function listing($dir) {
	$result = array();
	foreach (glob($dir.'/*') as $filename) {
		if(is_file($filename)) {
			$result[] = $filename;
		}
		else {
			
			$result = array_merge(listing($filename), $result);
		}
	}
	return $result;
}

This function accept one parameter - string, dir path. It will return a array, containing all files found in this and all its subfolders.

List files in folder

PHP