wp_resource_hints()

Prints resource hints to browsers for pre-fetching, pre-rendering and pre-connecting to web sites.


Description Description

Gives hints to browsers to prefetch specific pages or render them in the background, to perform DNS lookups or to begin the connection handshake (DNS, TCP, TLS) in the background.

These performance improving indicators work by using <link rel"…">.


Source Source

File: wp-includes/general-template.php

function wp_resource_hints() {
	$hints = array(
		'dns-prefetch' => wp_dependencies_unique_hosts(),
		'preconnect'   => array(),
		'prefetch'     => array(),
		'prerender'    => array(),
	);

	/*
	 * Add DNS prefetch for the Emoji CDN.
	 * The path is removed in the foreach loop below.
	 */
	/** This filter is documented in wp-includes/formatting.php */
	$hints['dns-prefetch'][] = apply_filters( 'emoji_svg_url', 'https://s.w.org/images/core/emoji/2.3/svg/' );

	foreach ( $hints as $relation_type => $urls ) {
		$unique_urls = array();

		/**
		 * Filters domains and URLs for resource hints of relation type.
		 *
		 * @since 4.6.0
		 *
		 * @param array  $urls          URLs to print for resource hints.
		 * @param string $relation_type The relation type the URLs are printed for, e.g. 'preconnect' or 'prerender'.
		 */
		$urls = apply_filters( 'wp_resource_hints', $urls, $relation_type );

		foreach ( $urls as $key => $url ) {
			$atts = array();

			if ( is_array( $url ) ) {
				if ( isset( $url['href'] ) ) {
					$atts = $url;
					$url  = $url['href'];
				} else {
					continue;
				}
			}

			$url = esc_url( $url, array( 'http', 'https' ) );

			if ( ! $url ) {
				continue;
			}

			if ( isset( $unique_urls[ $url ] ) ) {
				continue;
			}

			if ( in_array( $relation_type, array( 'preconnect', 'dns-prefetch' ) ) ) {
				$parsed = wp_parse_url( $url );

				if ( empty( $parsed['host'] ) ) {
					continue;
				}

				if ( 'preconnect' === $relation_type && ! empty( $parsed['scheme'] ) ) {
					$url = $parsed['scheme'] . '://' . $parsed['host'];
				} else {
					// Use protocol-relative URLs for dns-prefetch or if scheme is missing.
					$url = '//' . $parsed['host'];
				}
			}

			$atts['rel'] = $relation_type;
			$atts['href'] = $url;

			$unique_urls[ $url ] = $atts;
		}

		foreach ( $unique_urls as $atts ) {
			$html = '';

			foreach ( $atts as $attr => $value ) {
				if ( ! is_scalar( $value ) ||
				     ( ! in_array( $attr, array( 'as', 'crossorigin', 'href', 'pr', 'rel', 'type' ), true ) && ! is_numeric( $attr ))
				) {
					continue;
				}

				$value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );

				if ( ! is_string( $attr ) ) {
					$html .= " $value";
				} else {
					$html .= " $attr='$value'";
				}
			}

			$html = trim( $html );

			echo "<link $html />\n";
		}
	}
}

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Version Description
4.6.0 Introduced.


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