wp_make_content_images_responsive() WP 4.4.0

Filters 'img' elements in post content to add 'srcset' and 'sizes' attributes.

No Hooks.

Return

String. Converted content with 'srcset' and 'sizes' attributes added to images.

Usage

wp_make_content_images_responsive( $content );

$content(string) (required)

The raw post content to be filtered.

Code of wp make content images responsive: wp-includes/media.php ^{VER 5.1.1}

<?php
function wp_make_content_images_responsive( $content ) {
	if ( ! preg_match_all( '/<img [^>]+>/', $content, $matches ) ) {
		return $content;
	}

	$selected_images = $attachment_ids = array();

	foreach ( $matches[0] as $image ) {
		if ( false === strpos( $image, ' srcset=' ) && preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) &&
			( $attachment_id = absint( $class_id[1] ) ) ) {

			/*
			 * If exactly the same image tag is used more than once, overwrite it.
			 * All identical tags will be replaced later with 'str_replace()'.
			 */
			$selected_images[ $image ] = $attachment_id;
			// Overwrite the ID when the same image is included more than once.
			$attachment_ids[ $attachment_id ] = true;
		}
	}

	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( array_keys( $attachment_ids ), false, true );
	}

	foreach ( $selected_images as $image => $attachment_id ) {
		$image_meta = wp_get_attachment_metadata( $attachment_id );
		$content    = str_replace( $image, wp_image_add_srcset_and_sizes( $image, $image_meta, $attachment_id ), $content );
	}

	return $content;
}