WordPress at a glance
function is not described

wp_filter_content_tags() WP 5.5.0

Filters specific tags in post content and modifies their markup.

Modifies HTML tags in post content to include new browser and HTML technologies that may not have existed at the time of post creation. These modifications currently include adding srcset, sizes, and loading attributes to img HTML tags, as well as adding loading attributes to iframe HTML tags. Future similar optimizations should be added/expected here.

No Hooks.

Return

String. Converted content with images modified.

Usage

wp_filter_content_tags( $content, $context );
$content(string) (required)
The HTML content to be filtered.
$context(string)
Additional context to pass to the filters.
Default: current_filter() when not set

Notes

  • See: wp_img_tag_add_width_and_height_attr()
  • See: wp_img_tag_add_srcset_and_sizes_attr()
  • See: wp_img_tag_add_loading_attr()
  • See: wp_iframe_tag_add_loading_attr()

Changelog

Since 5.5.0 Introduced.
Since 5.7.0 Now supports adding loading attributes to iframe tags.

Code of wp_filter_content_tags() WP 5.7

<?php
function wp_filter_content_tags( $content, $context = null ) {
	if ( null === $context ) {
		$context = current_filter();
	}

	$add_img_loading_attr    = wp_lazy_loading_enabled( 'img', $context );
	$add_iframe_loading_attr = wp_lazy_loading_enabled( 'iframe', $context );

	if ( ! preg_match_all( '/<(img|iframe)\s[^>]+>/', $content, $matches, PREG_SET_ORDER ) ) {
		return $content;
	}

	// List of the unique `img` tags found in $content.
	$images = array();

	// List of the unique `iframe` tags found in $content.
	$iframes = array();

	foreach ( $matches as $match ) {
		list( $tag, $tag_name ) = $match;

		switch ( $tag_name ) {
			case 'img':
				if ( preg_match( '/wp-image-([0-9]+)/i', $tag, $class_id ) ) {
					$attachment_id = absint( $class_id[1] );

					if ( $attachment_id ) {
						// If exactly the same image tag is used more than once, overwrite it.
						// All identical tags will be replaced later with 'str_replace()'.
						$images[ $tag ] = $attachment_id;
						break;
					}
				}
				$images[ $tag ] = 0;
				break;
			case 'iframe':
				$iframes[ $tag ] = 0;
				break;
		}
	}

	// Reduce the array to unique attachment IDs.
	$attachment_ids = array_unique( array_filter( array_values( $images ) ) );

	if ( count( $attachment_ids ) > 1 ) {
		/*
		 * Warm the object cache with post and meta information for all found
		 * images to avoid making individual database calls.
		 */
		_prime_post_caches( $attachment_ids, false, true );
	}

	foreach ( $images as $image => $attachment_id ) {
		$filtered_image = $image;

		// Add 'width' and 'height' attributes if applicable.
		if ( $attachment_id > 0 && false === strpos( $filtered_image, ' width=' ) && false === strpos( $filtered_image, ' height=' ) ) {
			$filtered_image = wp_img_tag_add_width_and_height_attr( $filtered_image, $context, $attachment_id );
		}

		// Add 'srcset' and 'sizes' attributes if applicable.
		if ( $attachment_id > 0 && false === strpos( $filtered_image, ' srcset=' ) ) {
			$filtered_image = wp_img_tag_add_srcset_and_sizes_attr( $filtered_image, $context, $attachment_id );
		}

		// Add 'loading' attribute if applicable.
		if ( $add_img_loading_attr && false === strpos( $filtered_image, ' loading=' ) ) {
			$filtered_image = wp_img_tag_add_loading_attr( $filtered_image, $context );
		}

		if ( $filtered_image !== $image ) {
			$content = str_replace( $image, $filtered_image, $content );
		}
	}

	foreach ( $iframes as $iframe => $attachment_id ) {
		$filtered_iframe = $iframe;

		// Add 'loading' attribute if applicable.
		if ( $add_iframe_loading_attr && false === strpos( $filtered_iframe, ' loading=' ) ) {
			$filtered_iframe = wp_iframe_tag_add_loading_attr( $filtered_iframe, $context );
		}

		if ( $filtered_iframe !== $iframe ) {
			$content = str_replace( $iframe, $filtered_iframe, $content );
		}
	}

	return $content;
}