PHP simulates the function code of js function unescape

function unescape($str)
{
$ret = '';
$len = strlen($str);
 
for ($i = 0; $i < $len; $i++)
{
if ($str[$i] == &#39;%&#39; && $str[$i+1] == &#39;u&#39;)
{
$val = hexdec(substr($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] == &#39;%&#39;)
{
$ret .= urldecode(substr($str, $i, 3));
$i += 2;
}
else $ret .= $str[$i];
}
return $ret;
}