PHP simulates the function code of js function unescape_PHP tutorial

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;
}