public static function genRandomStr($l) {
		$p = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
		$s = '';
		for ($i = 0; $i < $l; $i++) {
			$s .= $p[mt_rand(0, (mb_strlen($p) - 1))];
		}
		return $s;
	}