MD5-hash-like, Hexadecimal encrypt / decrypt funtion for PHP

encrypt-decrypt.php

<?php

echo encrypt('Hello','f03bcafdcefd43da');
echo decrypt('427bc6d8058613003a5f2f575f269465','f03bcafdcefd43da');

// Based on Nanhe Kumar answer : https://stackoverflow.com/a/52253745/4013321

function encrypt($plainText, $key) {
      $secretKey = hextobin(md5($key));
      $initVector = pack("C*", 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f);
      $openMode = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', 'cbc', '');
      $blockSize = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, 'cbc');
      $plainPad = pkcs5_pad($plainText, $blockSize);
      if (mcrypt_generic_init($openMode, $secretKey, $initVector) != -1) {
          $encryptedText = mcrypt_generic($openMode, $plainPad);
          mcrypt_generic_deinit($openMode);
      }
    return bin2hex($encryptedText);
}

function decrypt($encryptedText, $key) {
      $secretKey = hextobin(md5($key));
      $initVector = pack("C*", 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f);
      $encryptedText = hextobin($encryptedText);
      $openMode = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', 'cbc', '');
      mcrypt_generic_init($openMode, $secretKey, $initVector);
      $decryptedText = mdecrypt_generic($openMode, $encryptedText);
      $decryptedText = rtrim($decryptedText, "\0");
      mcrypt_generic_deinit($openMode);
    return $decryptedText;
}

//*********** Padding Function *********************
  function pkcs5_pad($plainText, $blockSize) {
    $pad = $blockSize - (strlen($plainText) % $blockSize);
    return $plainText . str_repeat(chr($pad), $pad);
}

//********** Hexadecimal to Binary function for php 4.0 version ********
  function hextobin($hexString) {
    $length = strlen($hexString);
    $binString = "";
    $count = 0;
    while ($count < $length) {
        $subString = substr($hexString, $count, 2);
        $packedString = pack("H*", $subString);
        if ($count == 0) {
            $binString = $packedString;
        } else {
            $binString .= $packedString;
        }

        $count += 2;
    }
    return $binString;
}

?>