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// Get all image src from HTML content
function my_get_images_from_content( $html ) {
    $doc = new DOMDocument();
    @$doc->loadHTML( $html );
    $imageTags = $doc->getElementsByTagName( 'img' );
    $all_images = array();

    foreach( $imageTags as $tag ) {
        $all_images[] = $tag->getAttribute( 'src' );
    }

    return implode( ",", $all_images );
}