redirect_canonical( string $requested_url = null, bool $do_redirect = true )

Redirects incoming links to the proper URL based on the site url.


Description Description

Search engines consider www.somedomain.com and somedomain.com to be two different URLs when they both go to the same location. This SEO enhancement prevents penalty for duplicate content by redirecting all incoming links to one or the other.

Prevents redirection for feeds, trackbacks, searches, and admin URLs. Does not redirect on non-pretty-permalink-supporting IIS 7+, page/post previews, WP admin, Trackbacks, robots.txt, favicon.ico, searches, or on POST requests.

Will also attempt to find the correct link when a user enters a URL that does not exist based on exact WordPress query. Will instead try to parse the URL or query in an attempt to figure the correct page to go to.


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Parameters Parameters

$requested_url

(string) (Optional) The URL that was requested, used to figure if redirect is needed.

Default value: null

$do_redirect

(bool) (Optional) Redirect to the new URL.

Default value: true


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Return Return

(string|void) The string of the URL, if redirect needed.


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Source Source

File: wp-includes/canonical.php

function redirect_canonical( $requested_url = null, $do_redirect = true ) {
	global $wp_rewrite, $is_IIS, $wp_query, $wpdb, $wp;

	if ( isset( $_SERVER['REQUEST_METHOD'] ) && ! in_array( strtoupper( $_SERVER['REQUEST_METHOD'] ), array( 'GET', 'HEAD' ), true ) ) {
		return;
	}

	// If we're not in wp-admin and the post has been published and preview nonce
	// is non-existent or invalid then no need for preview in query.
	if ( is_preview() && get_query_var( 'p' ) && 'publish' === get_post_status( get_query_var( 'p' ) ) ) {
		if ( ! isset( $_GET['preview_id'] )
			|| ! isset( $_GET['preview_nonce'] )
			|| ! wp_verify_nonce( $_GET['preview_nonce'], 'post_preview_' . (int) $_GET['preview_id'] )
		) {
			$wp_query->is_preview = false;
		}
	}

	if ( is_admin() || is_search() || is_preview() || is_trackback() || is_favicon()
		|| ( $is_IIS && ! iis7_supports_permalinks() )
	) {
		return;
	}

	if ( ! $requested_url && isset( $_SERVER['HTTP_HOST'] ) ) {
		// Build the URL in the address bar.
		$requested_url  = is_ssl() ? 'https://' : 'http://';
		$requested_url .= $_SERVER['HTTP_HOST'];
		$requested_url .= $_SERVER['REQUEST_URI'];
	}

	$original = parse_url( $requested_url );
	if ( false === $original ) {
		return;
	}

	$redirect     = $original;
	$redirect_url = false;

	// Notice fixing.
	if ( ! isset( $redirect['path'] ) ) {
		$redirect['path'] = '';
	}
	if ( ! isset( $redirect['query'] ) ) {
		$redirect['query'] = '';
	}

	/*
	 * If the original URL ended with non-breaking spaces, they were almost
	 * certainly inserted by accident. Let's remove them, so the reader doesn't
	 * see a 404 error with no obvious cause.
	 */
	$redirect['path'] = preg_replace( '|(%C2%A0)+$|i', '', $redirect['path'] );

	// It's not a preview, so remove it from URL.
	if ( get_query_var( 'preview' ) ) {
		$redirect['query'] = remove_query_arg( 'preview', $redirect['query'] );
	}

	$post_id = get_query_var( 'p' );

	if ( is_feed() && $post_id ) {
		$redirect_url = get_post_comments_feed_link( $post_id, get_query_var( 'feed' ) );

		if ( $redirect_url ) {
			$redirect['query'] = _remove_qs_args_if_not_in_url(
				$redirect['query'],
				array( 'p', 'page_id', 'attachment_id', 'pagename', 'name', 'post_type', 'feed' ),
				$redirect_url
			);

			$redirect['path'] = parse_url( $redirect_url, PHP_URL_PATH );
		}
	}

	if ( is_singular() && $wp_query->post_count < 1 && $post_id ) {

		$vars = $wpdb->get_results( $wpdb->prepare( "SELECT post_type, post_parent FROM $wpdb->posts WHERE ID = %d", $post_id ) );

		if ( ! empty( $vars[0] ) ) {
			$vars = $vars[0];

			if ( 'revision' === $vars->post_type && $vars->post_parent > 0 ) {
				$post_id = $vars->post_parent;
			}

			$redirect_url = get_permalink( $post_id );

			if ( $redirect_url ) {
				$redirect['query'] = _remove_qs_args_if_not_in_url(
					$redirect['query'],
					array( 'p', 'page_id', 'attachment_id', 'pagename', 'name', 'post_type' ),
					$redirect_url
				);
			}
		}
	}

	// These tests give us a WP-generated permalink.
	if ( is_404() ) {

		// Redirect ?page_id, ?p=, ?attachment_id= to their respective URLs.
		$post_id = max( get_query_var( 'p' ), get_query_var( 'page_id' ), get_query_var( 'attachment_id' ) );

		$redirect_post = $post_id ? get_post( $post_id ) : false;

		if ( $redirect_post ) {
			$post_type_obj = get_post_type_object( $redirect_post->post_type );

			if ( $post_type_obj->public && 'auto-draft' !== $redirect_post->post_status ) {
				$redirect_url = get_permalink( $redirect_post );

				$redirect['query'] = _remove_qs_args_if_not_in_url(
					$redirect['query'],
					array( 'p', 'page_id', 'attachment_id', 'pagename', 'name', 'post_type' ),
					$redirect_url
				);
			}
		}

		$year  = get_query_var( 'year' );
		$month = get_query_var( 'monthnum' );
		$day   = get_query_var( 'day' );

		if ( $year && $month && $day ) {
			$date = sprintf( '%04d-%02d-%02d', $year, $month, $day );

			if ( ! wp_checkdate( $month, $day, $year, $date ) ) {
				$redirect_url = get_month_link( $year, $month );

				$redirect['query'] = _remove_qs_args_if_not_in_url(
					$redirect['query'],
					array( 'year', 'monthnum', 'day' ),
					$redirect_url
				);
			}
		} elseif ( $year && $month && $month > 12 ) {
			$redirect_url = get_year_link( $year );

			$redirect['query'] = _remove_qs_args_if_not_in_url(
				$redirect['query'],
				array( 'year', 'monthnum' ),
				$redirect_url
			);
		}

		// Strip off non-existing <!--nextpage--> links from single posts or pages.
		if ( get_query_var( 'page' ) ) {
			$post_id = 0;

			if ( $wp_query->queried_object instanceof WP_Post ) {
				$post_id = $wp_query->queried_object->ID;
			} elseif ( $wp_query->post ) {
				$post_id = $wp_query->post->ID;
			}

			if ( $post_id ) {
				$redirect_url = get_permalink( $post_id );

				$redirect['path']  = rtrim( $redirect['path'], (int) get_query_var( 'page' ) . '/' );
				$redirect['query'] = remove_query_arg( 'page', $redirect['query'] );
			}
		}

		if ( ! $redirect_url ) {
			$redirect_url = redirect_guess_404_permalink();

			if ( $redirect_url ) {
				$redirect['query'] = _remove_qs_args_if_not_in_url(
					$redirect['query'],
					array( 'page', 'feed', 'p', 'page_id', 'attachment_id', 'pagename', 'name', 'post_type' ),
					$redirect_url
				);
			}
		}
	} elseif ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks() ) {

		// Rewriting of old ?p=X, ?m=2004, ?m=200401, ?m=20040101.
		if ( is_attachment()
			&& ! array_diff( array_keys( $wp->query_vars ), array( 'attachment', 'attachment_id' ) )
			&& ! $redirect_url
		) {
			if ( ! empty( $_GET['attachment_id'] ) ) {
				$redirect_url = get_attachment_link( get_query_var( 'attachment_id' ) );

				if ( $redirect_url ) {
					$redirect['query'] = remove_query_arg( 'attachment_id', $redirect['query'] );
				}
			} else {
				$redirect_url = get_attachment_link();
			}
		} elseif ( is_single() && ! empty( $_GET['p'] ) && ! $redirect_url ) {
			$redirect_url = get_permalink( get_query_var( 'p' ) );

			if ( $redirect_url ) {
				$redirect['query'] = remove_query_arg( array( 'p', 'post_type' ), $redirect['query'] );
			}
		} elseif ( is_single() && ! empty( $_GET['name'] ) && ! $redirect_url ) {
			$redirect_url = get_permalink( $wp_query->get_queried_object_id() );

			if ( $redirect_url ) {
				$redirect['query'] = remove_query_arg( 'name', $redirect['query'] );
			}
		} elseif ( is_page() && ! empty( $_GET['page_id'] ) && ! $redirect_url ) {
			$redirect_url = get_permalink( get_query_var( 'page_id' ) );

			if ( $redirect_url ) {
				$redirect['query'] = remove_query_arg( 'page_id', $redirect['query'] );
			}
		} elseif ( is_page() && ! is_feed() && ! $redirect_url
			&& 'page' === get_option( 'show_on_front' ) && get_queried_object_id() === (int) get_option( 'page_on_front' )
		) {
			$redirect_url = home_url( '/' );
		} elseif ( is_home() && ! empty( $_GET['page_id'] ) && ! $redirect_url
			&& 'page' === get_option( 'show_on_front' ) && get_query_var( 'page_id' ) === (int) get_option( 'page_for_posts' )
		) {
			$redirect_url = get_permalink( get_option( 'page_for_posts' ) );

			if ( $redirect_url ) {
				$redirect['query'] = remove_query_arg( 'page_id', $redirect['query'] );
			}
		} elseif ( ! empty( $_GET['m'] ) && ( is_year() || is_month() || is_day() ) ) {
			$m = get_query_var( 'm' );

			switch ( strlen( $m ) ) {
				case 4: // Yearly.
					$redirect_url = get_year_link( $m );
					break;
				case 6: // Monthly.
					$redirect_url = get_month_link( substr( $m, 0, 4 ), substr( $m, 4, 2 ) );
					break;
				case 8: // Daily.
					$redirect_url = get_day_link( substr( $m, 0, 4 ), substr( $m, 4, 2 ), substr( $m, 6, 2 ) );
					break;
			}

			if ( $redirect_url ) {
				$redirect['query'] = remove_query_arg( 'm', $redirect['query'] );
			}
			// Now moving on to non ?m=X year/month/day links.
		} elseif ( is_date() ) {
			$year  = get_query_var( 'year' );
			$month = get_query_var( 'monthnum' );
			$day   = get_query_var( 'day' );

			if ( is_day() && $year && $month && ! empty( $_GET['day'] ) ) {
				$redirect_url = get_day_link( $year, $month, $day );

				if ( $redirect_url ) {
					$redirect['query'] = remove_query_arg( array( 'year', 'monthnum', 'day' ), $redirect['query'] );
				}
			} elseif ( is_month() && $year && ! empty( $_GET['monthnum'] ) ) {
				$redirect_url = get_month_link( $year, $month );

				if ( $redirect_url ) {
					$redirect['query'] = remove_query_arg( array( 'year', 'monthnum' ), $redirect['query'] );
				}
			} elseif ( is_year() && ! empty( $_GET['year'] ) ) {
				$redirect_url = get_year_link( $year );

				if ( $redirect_url ) {
					$redirect['query'] = remove_query_arg( 'year', $redirect['query'] );
				}
			}
		} elseif ( is_author() && ! empty( $_GET['author'] ) && preg_match( '|^[0-9]+$|', $_GET['author'] ) ) {
			$author = get_userdata( get_query_var( 'author' ) );

			if ( false !== $author
				&& $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM $wpdb->posts WHERE $wpdb->posts.post_author = %d AND $wpdb->posts.post_status = 'publish' LIMIT 1", $author->ID ) )
			) {
				$redirect_url = get_author_posts_url( $author->ID, $author->user_nicename );

				if ( $redirect_url ) {
					$redirect['query'] = remove_query_arg( 'author', $redirect['query'] );
				}
			}
		} elseif ( is_category() || is_tag() || is_tax() ) { // Terms (tags/categories).
			$term_count = 0;

			foreach ( $wp_query->tax_query->queried_terms as $tax_query ) {
				$term_count += count( $tax_query['terms'] );
			}

			$obj = $wp_query->get_queried_object();

			if ( $term_count <= 1 && ! empty( $obj->term_id ) ) {
				$tax_url = get_term_link( (int) $obj->term_id, $obj->taxonomy );

				if ( $tax_url && ! is_wp_error( $tax_url ) ) {
					if ( ! empty( $redirect['query'] ) ) {
						// Strip taxonomy query vars off the URL.
						$qv_remove = array( 'term', 'taxonomy' );

						if ( is_category() ) {
							$qv_remove[] = 'category_name';
							$qv_remove[] = 'cat';
						} elseif ( is_tag() ) {
							$qv_remove[] = 'tag';
							$qv_remove[] = 'tag_id';
						} else {
							// Custom taxonomies will have a custom query var, remove those too.
							$tax_obj = get_taxonomy( $obj->taxonomy );
							if ( false !== $tax_obj->query_var ) {
								$qv_remove[] = $tax_obj->query_var;
							}
						}

						$rewrite_vars = array_diff( array_keys( $wp_query->query ), array_keys( $_GET ) );

						// Check to see if all the query vars are coming from the rewrite, none are set via $_GET.
						if ( ! array_diff( $rewrite_vars, array_keys( $_GET ) ) ) {
							// Remove all of the per-tax query vars.
							$redirect['query'] = remove_query_arg( $qv_remove, $redirect['query'] );

							// Create the destination URL for this taxonomy.
							$tax_url = parse_url( $tax_url );

							if ( ! empty( $tax_url['query'] ) ) {
								// Taxonomy accessible via ?taxonomy=...&term=... or any custom query var.
								parse_str( $tax_url['query'], $query_vars );
								$redirect['query'] = add_query_arg( $query_vars, $redirect['query'] );
							} else {
								// Taxonomy is accessible via a "pretty URL".
								$redirect['path'] = $tax_url['path'];
							}
						} else {
							// Some query vars are set via $_GET. Unset those from $_GET that exist via the rewrite.
							foreach ( $qv_remove as $_qv ) {
								if ( isset( $rewrite_vars[ $_qv ] ) ) {
									$redirect['query'] = remove_query_arg( $_qv, $redirect['query'] );
								}
							}
						}
					}
				}
			}
		} elseif ( is_single() && strpos( $wp_rewrite->permalink_structure, '%category%' ) !== false ) {
			$category_name = get_query_var( 'category_name' );

			if ( $category_name ) {
				$category = get_category_by_path( $category_name );

				if ( ! $category || is_wp_error( $category )
					|| ! has_term( $category->term_id, 'category', $wp_query->get_queried_object_id() )
				) {
					$redirect_url = get_permalink( $wp_query->get_queried_object_id() );
				}
			}
		}

		// Post paging.
		if ( is_singular() && get_query_var( 'page' ) ) {
			$page = get_query_var( 'page' );

			if ( ! $redirect_url ) {
				$redirect_url = get_permalink( get_queried_object_id() );
			}

			if ( $page > 1 ) {
				$redirect_url = trailingslashit( $redirect_url );

				if ( is_front_page() ) {
					$redirect_url .= user_trailingslashit( "$wp_rewrite->pagination_base/$page", 'paged' );
				} else {
					$redirect_url .= user_trailingslashit( $page, 'single_paged' );
				}
			}

			$redirect['query'] = remove_query_arg( 'page', $redirect['query'] );
		}

		// Paging and feeds.
		if ( get_query_var( 'paged' ) || is_feed() || get_query_var( 'cpage' ) ) {
			$paged = get_query_var( 'paged' );
			$feed  = get_query_var( 'feed' );
			$cpage = get_query_var( 'cpage' );

			while ( preg_match( "#/$wp_rewrite->pagination_base/?[0-9]+?(/+)?$#", $redirect['path'] )
				|| preg_match( '#/(comments/?)?(feed|rss2?|rdf|atom)(/+)?$#', $redirect['path'] )
				|| preg_match( "#/{$wp_rewrite->comments_pagination_base}-[0-9]+(/+)?$#", $redirect['path'] )
			) {
				// Strip off any existing paging.
				$redirect['path'] = preg_replace( "#/$wp_rewrite->pagination_base/?[0-9]+?(/+)?$#", '/', $redirect['path'] );
				// Strip off feed endings.
				$redirect['path'] = preg_replace( '#/(comments/?)?(feed|rss2?|rdf|atom)(/+|$)#', '/', $redirect['path'] );
				// Strip off any existing comment paging.
				$redirect['path'] = preg_replace( "#/{$wp_rewrite->comments_pagination_base}-[0-9]+?(/+)?$#", '/', $redirect['path'] );
			}

			$addl_path    = '';
			$default_feed = get_default_feed();

			if ( is_feed() && in_array( $feed, $wp_rewrite->feeds, true ) ) {
				$addl_path = ! empty( $addl_path ) ? trailingslashit( $addl_path ) : '';

				if ( ! is_singular() && get_query_var( 'withcomments' ) ) {
					$addl_path .= 'comments/';
				}

				if ( ( 'rss' === $default_feed && 'feed' === $feed ) || 'rss' === $feed ) {
					$format = ( 'rss2' === $default_feed ) ? '' : 'rss2';
				} else {
					$format = ( $default_feed === $feed || 'feed' === $feed ) ? '' : $feed;
				}

				$addl_path .= user_trailingslashit( 'feed/' . $format, 'feed' );

				$redirect['query'] = remove_query_arg( 'feed', $redirect['query'] );
			} elseif ( is_feed() && 'old' === $feed ) {
				$old_feed_files = array(
					'wp-atom.php'         => 'atom',
					'wp-commentsrss2.php' => 'comments_rss2',
					'wp-feed.php'         => $default_feed,
					'wp-rdf.php'          => 'rdf',
					'wp-rss.php'          => 'rss2',
					'wp-rss2.php'         => 'rss2',
				);

				if ( isset( $old_feed_files[ basename( $redirect['path'] ) ] ) ) {
					$redirect_url = get_feed_link( $old_feed_files[ basename( $redirect['path'] ) ] );

					wp_redirect( $redirect_url, 301 );
					die();
				}
			}

			if ( $paged > 0 ) {
				$redirect['query'] = remove_query_arg( 'paged', $redirect['query'] );

				if ( ! is_feed() ) {
					if ( ! is_single() ) {
						$addl_path = ! empty( $addl_path ) ? trailingslashit( $addl_path ) : '';

						if ( $paged > 1 ) {
							$addl_path .= user_trailingslashit( "$wp_rewrite->pagination_base/$paged", 'paged' );
						}
					}
				} elseif ( $paged > 1 ) {
					$redirect['query'] = add_query_arg( 'paged', $paged, $redirect['query'] );
				}
			}

			$default_comments_page = get_option( 'default_comments_page' );

			if ( get_option( 'page_comments' )
				&& ( 'newest' === $default_comments_page && $cpage > 0
					|| 'newest' !== $default_comments_page && $cpage > 1 )
			) {
				$addl_path  = ( ! empty( $addl_path ) ? trailingslashit( $addl_path ) : '' );
				$addl_path .= user_trailingslashit( $wp_rewrite->comments_pagination_base . '-' . $cpage, 'commentpaged' );

				$redirect['query'] = remove_query_arg( 'cpage', $redirect['query'] );
			}

			// Strip off trailing /index.php/.
			$redirect['path'] = preg_replace( '|/' . preg_quote( $wp_rewrite->index, '|' ) . '/?$|', '/', $redirect['path'] );
			$redirect['path'] = user_trailingslashit( $redirect['path'] );

			if ( ! empty( $addl_path )
				&& $wp_rewrite->using_index_permalinks()
				&& strpos( $redirect['path'], '/' . $wp_rewrite->index . '/' ) === false
			) {
				$redirect['path'] = trailingslashit( $redirect['path'] ) . $wp_rewrite->index . '/';
			}

			if ( ! empty( $addl_path ) ) {
				$redirect['path'] = trailingslashit( $redirect['path'] ) . $addl_path;
			}

			// Remove trailing slash for sitemaps requests.
			if ( ! empty( get_query_var( 'sitemap' ) ) ) {
				$redirect['path'] = untrailingslashit( $redirect['path'] );
			}

			$redirect_url = $redirect['scheme'] . '://' . $redirect['host'] . $redirect['path'];
		}

		if ( 'wp-register.php' === basename( $redirect['path'] ) ) {
			if ( is_multisite() ) {
				/** This filter is documented in wp-login.php */
				$redirect_url = apply_filters( 'wp_signup_location', network_site_url( 'wp-signup.php' ) );
			} else {
				$redirect_url = wp_registration_url();
			}

			wp_redirect( $redirect_url, 301 );
			die();
		}
	}

	$redirect['query'] = preg_replace( '#^\??&*?#', '', $redirect['query'] );

	// Tack on any additional query vars.
	if ( $redirect_url && ! empty( $redirect['query'] ) ) {
		parse_str( $redirect['query'], $_parsed_query );
		$redirect = parse_url( $redirect_url );

		if ( ! empty( $_parsed_query['name'] ) && ! empty( $redirect['query'] ) ) {
			parse_str( $redirect['query'], $_parsed_redirect_query );

			if ( empty( $_parsed_redirect_query['name'] ) ) {
				unset( $_parsed_query['name'] );
			}
		}

		$_parsed_query = array_combine(
			rawurlencode_deep( array_keys( $_parsed_query ) ),
			rawurlencode_deep( array_values( $_parsed_query ) )
		);

		$redirect_url = add_query_arg( $_parsed_query, $redirect_url );
	}

	if ( $redirect_url ) {
		$redirect = parse_url( $redirect_url );
	}

	// www.example.com vs. example.com
	$user_home = parse_url( home_url() );

	if ( ! empty( $user_home['host'] ) ) {
		$redirect['host'] = $user_home['host'];
	}

	if ( empty( $user_home['path'] ) ) {
		$user_home['path'] = '/';
	}

	// Handle ports.
	if ( ! empty( $user_home['port'] ) ) {
		$redirect['port'] = $user_home['port'];
	} else {
		unset( $redirect['port'] );
	}

	// Trailing /index.php.
	$redirect['path'] = preg_replace( '|/' . preg_quote( $wp_rewrite->index, '|' ) . '/*?$|', '/', $redirect['path'] );

	$punctuation_pattern = implode(
		'|',
		array_map(
			'preg_quote',
			array(
				' ',
				'%20',  // Space.
				'!',
				'%21',  // Exclamation mark.
				'"',
				'%22',  // Double quote.
				"'",
				'%27',  // Single quote.
				'(',
				'%28',  // Opening bracket.
				')',
				'%29',  // Closing bracket.
				',',
				'%2C',  // Comma.
				'.',
				'%2E',  // Period.
				';',
				'%3B',  // Semicolon.
				'{',
				'%7B',  // Opening curly bracket.
				'}',
				'%7D',  // Closing curly bracket.
				'%E2%80%9C', // Opening curly quote.
				'%E2%80%9D', // Closing curly quote.
			)
		)
	);

	// Remove trailing spaces and end punctuation from the path.
	$redirect['path'] = preg_replace( "#($punctuation_pattern)+$#", '', $redirect['path'] );

	if ( ! empty( $redirect['query'] ) ) {
		// Remove trailing spaces and end punctuation from certain terminating query string args.
		$redirect['query'] = preg_replace( "#((^|&)(p|page_id|cat|tag)=[^&]*?)($punctuation_pattern)+$#", '$1', $redirect['query'] );

		// Clean up empty query strings.
		$redirect['query'] = trim( preg_replace( '#(^|&)(p|page_id|cat|tag)=?(&|$)#', '&', $redirect['query'] ), '&' );

		// Redirect obsolete feeds.
		$redirect['query'] = preg_replace( '#(^|&)feed=rss(&|$)#', '$1feed=rss2$2', $redirect['query'] );

		// Remove redundant leading ampersands.
		$redirect['query'] = preg_replace( '#^\??&*?#', '', $redirect['query'] );
	}

	// Strip /index.php/ when we're not using PATHINFO permalinks.
	if ( ! $wp_rewrite->using_index_permalinks() ) {
		$redirect['path'] = str_replace( '/' . $wp_rewrite->index . '/', '/', $redirect['path'] );
	}

	// Trailing slashes.
	if ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks()
		&& ! is_404() && ( ! is_front_page() || is_front_page() && get_query_var( 'paged' ) > 1 )
	) {
		$user_ts_type = '';

		if ( get_query_var( 'paged' ) > 0 ) {
			$user_ts_type = 'paged';
		} else {
			foreach ( array( 'single', 'category', 'page', 'day', 'month', 'year', 'home' ) as $type ) {
				$func = 'is_' . $type;
				if ( call_user_func( $func ) ) {
					$user_ts_type = $type;
					break;
				}
			}
		}

		$redirect['path'] = user_trailingslashit( $redirect['path'], $user_ts_type );
	} elseif ( is_front_page() ) {
		$redirect['path'] = trailingslashit( $redirect['path'] );
	}

	// Remove trailing slash for robots.txt or sitemap requests.
	if ( is_robots()
		|| ! empty( get_query_var( 'sitemap' ) ) || ! empty( get_query_var( 'sitemap-stylesheet' ) )
	) {
		$redirect['path'] = untrailingslashit( $redirect['path'] );
	}

	// Strip multiple slashes out of the URL.
	if ( strpos( $redirect['path'], '//' ) > -1 ) {
		$redirect['path'] = preg_replace( '|/+|', '/', $redirect['path'] );
	}

	// Always trailing slash the Front Page URL.
	if ( trailingslashit( $redirect['path'] ) === trailingslashit( $user_home['path'] ) ) {
		$redirect['path'] = trailingslashit( $redirect['path'] );
	}

	$original_host_low = strtolower( $original['host'] );
	$redirect_host_low = strtolower( $redirect['host'] );

	// Ignore differences in host capitalization, as this can lead to infinite redirects.
	// Only redirect no-www <=> yes-www.
	if ( $original_host_low === $redirect_host_low
		|| ( 'www.' . $original_host_low !== $redirect_host_low
			&& 'www.' . $redirect_host_low !== $original_host_low )
	) {
		$redirect['host'] = $original['host'];
	}

	$compare_original = array( $original['host'], $original['path'] );

	if ( ! empty( $original['port'] ) ) {
		$compare_original[] = $original['port'];
	}

	if ( ! empty( $original['query'] ) ) {
		$compare_original[] = $original['query'];
	}

	$compare_redirect = array( $redirect['host'], $redirect['path'] );

	if ( ! empty( $redirect['port'] ) ) {
		$compare_redirect[] = $redirect['port'];
	}

	if ( ! empty( $redirect['query'] ) ) {
		$compare_redirect[] = $redirect['query'];
	}

	if ( $compare_original !== $compare_redirect ) {
		$redirect_url = $redirect['scheme'] . '://' . $redirect['host'];

		if ( ! empty( $redirect['port'] ) ) {
			$redirect_url .= ':' . $redirect['port'];
		}

		$redirect_url .= $redirect['path'];

		if ( ! empty( $redirect['query'] ) ) {
			$redirect_url .= '?' . $redirect['query'];
		}
	}

	if ( ! $redirect_url || $redirect_url === $requested_url ) {
		return;
	}

	// Hex encoded octets are case-insensitive.
	if ( false !== strpos( $requested_url, '%' ) ) {
		if ( ! function_exists( 'lowercase_octets' ) ) {
			/**
			 * Converts the first hex-encoded octet match to lowercase.
			 *
			 * @since 3.1.0
			 * @ignore
			 *
			 * @param array $matches Hex-encoded octet matches for the requested URL.
			 * @return string Lowercased version of the first match.
			 */
			function lowercase_octets( $matches ) {
				return strtolower( $matches[0] );
			}
		}

		$requested_url = preg_replace_callback( '|%[a-fA-F0-9][a-fA-F0-9]|', 'lowercase_octets', $requested_url );
	}

	/**
	 * Filters the canonical redirect URL.
	 *
	 * Returning false to this filter will cancel the redirect.
	 *
	 * @since 2.3.0
	 *
	 * @param string $redirect_url  The redirect URL.
	 * @param string $requested_url The requested URL.
	 */
	$redirect_url = apply_filters( 'redirect_canonical', $redirect_url, $requested_url );

	// Yes, again -- in case the filter aborted the request.
	if ( ! $redirect_url || strip_fragment_from_url( $redirect_url ) === strip_fragment_from_url( $requested_url ) ) {
		return;
	}

	if ( $do_redirect ) {
		// Protect against chained redirects.
		if ( ! redirect_canonical( $redirect_url, false ) ) {
			wp_redirect( $redirect_url, 301 );
			exit;
		} else {
			// Debug.
			// die("1: $redirect_url<br />2: " . redirect_canonical( $redirect_url, false ) );
			return;
		}
	} else {
		return $redirect_url;
	}
}


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Changelog
Version Description
2.3.0 Introduced.

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User Contributed Notes User Contributed Notes

  1. Skip to note 1 content
    Contributed by Khoi Pro

    Remove Redirect for public var like ‘year’ in Custom Taxonomy.

    For example, a link: /?event_category=xxx?year=2017
    If you don’t add this filter:
    – It redirects to /2016/
    If you use this redirect:
    – It remains at page /?event_category=xxx

    This code is helpful for creating a filter in Taxonomies.

        add_filter( 'redirect_canonical', 'pick_event_year_redirect', 10 );
    	function pick_event_year_redirect( $redirect_url ) {
    		if ( is_tax( 'event_category' ) && is_year() ) {
    			return '';
    		}
    
    		return $redirect_url;
    	}

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