Change src inside iframe tag

// $video_HTML - string with iframe tag
preg_match('/src="(.+?)"/', $video_HTML, $matches);
$src = $matches[1];
// Add extra parameters to src and replace HTML.
$params = array(
    'controls'  => 1,
    'hd'        => 1,
    'title'     => 0,
    'portrait'  => 0,
    'byline'    => 0,
    'autoplay'  => 1
);
$new_src = add_query_arg($params, $src);
$video_HTML = str_replace($src, $new_src, $video_HTML);

$video_HTML = str_replace("src", "data-src", $video_HTML);