php模拟js函数unescape的函数代码

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; 
}