php解码escape

function unescape($str){ 
			$ret = ''; 
			$len = strlen($str); 
			for ($i = 0; $i < $len; $i++){ 
			if ($str[$i] == '%' && $str[$i+1] == 'u'){ 
			$val = hexdec(substr($str, $i+2, 4)); 
			if ($val < 0x7f) $ret .= chr($val); 
			else if($val < 0x800) $ret .= chr(0xc0|($val>>6)).chr(0x80|($val&0x3f)); 
			else $ret .= chr(0xe0|($val>>12)).chr(0x80|(($val>>6)&0x3f)).chr(0x80|($val&0x3f)); 
			$i += 5; 
		}else if ($str[$i] == '%'){ 
			$ret .= urldecode(substr($str, $i, 3)); 
			$i += 2; 
		} 
			else $ret .= $str[$i]; 
		} 
			return $ret; 
		}