string $html, int $post_id, int $post_thumbnail_id, string|int $size, string|array $attr )
Filters the post thumbnail HTML.
The post thumbnail HTML.
The post ID.
The post thumbnail ID, or 0 if there isn't one.
Requested image size. Can be any registered image size name, or an array of width and height values in pixels (in that order).
Query string or array of attributes.
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return apply_filters( 'post_thumbnail_html', $html, $post->ID, $post_thumbnail_id, $size, $attr );
As of WP 5.9 (as far as I can tell), the
$attrparameter indicates it accepts a
array. If you are using parameter-typing in your methods/functions, and you previously indicated that your function should expect a string, you will see fatal errors due to this (as arrays are also sent, now). The change appears to have occurred here.
Add title attribute to post thumbnail: