$string = file_get_contents('a.txt');

$regex = '/b(http|rtmp|rtsp|mmsh|mms)://[-A-Z0-9+&@#/%?=~_|$!:,.;]*[A-Z0-9+&@#/%=~_|$]/i';
preg_match_all($regex, $string, $matches);
$urls = $matches[0];