PHP带符号十六进制转十进制

$hexString = "fffd";

$binNumeric = hexdec($hexString);

$binString = decbin($binNumeric - 1);

$str = '';
for ($i = 0; $i < strlen($binString); $i++) {
    $str .= $binString[$i] ^ 1;
}

var_dump(-bindec($str));