<?php

    $toDecode = "Hello,\\x20World\\x21\\x20How\\x20are\\x20you\\x3F"; //file_get_contents('hex.txt');

    function escapedHexToHex($escaped)
    {
      // return 'OK!';
      return chr(hexdec($escaped[1]));
    }

    $decoded = preg_replace_callback('/\\\\x([a-f0-9]{2})/i','escapedHexToHex', $toDecode);

    echo $decoded; //file_put_contents('haha.txt', $decoded);