Understanding some php code

functions.php

<?php
function encrypt($plainText,$key) {
   // md5 hash of $key, then output is a hex number
   // convert hex output of md5() to binary, but I think it should be "hex2bin"
  $key = hextobin(md5($key));

  // initialisation vector needed for decryption later too
  // packs data into binary string, treat data to be in unsigned char format. https://www.php.net/manual/en/function.pack.php
  $initVector = pack("C*", 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f);
  // generate encypted data
  // https://stackoverflow.com/questions/34871579/how-to-encrypt-plaintext-with-aes-256-cbc-in-php-using-openssl/34871988#34871988
  $openMode = openssl_encrypt($plainText, 'AES-128-CBC', $key, OPENSSL_RAW_DATA, $initVector);
  // convert output of openssl to hexadecimal
  $encryptedText = bin2hex($openMode);

  // return encrypted text
  return $encryptedText; 
}


// exactly the reverse of encrypt function above
function decrypt($encryptedText,$key) {

  $key = hextobin(md5($key));
  $initVector = pack("C*", 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f);
  $encryptedText = hextobin($encryptedText);
  $decryptedText = openssl_decrypt($encryptedText, 'AES-128-CBC', $key, OPENSSL_RAW_DATA, $initVector);
  return $decryptedText; 
}

function pkcs5_pad ($plainText, $blockSize) {
  // find how many characters the text is short for a multiple of block size
  $pad = $blockSize - (strlen($plainText) % $blockSize);
  // repeat the character as many times as needed to have the length be equal to a a block size multiple
  return $plainText . str_repeat(chr($pad), $pad);
}

// byte by byte conversion to binary
// not completely sure about why they aren't using hex2bin
// probably they want the bytewise conversion not number conversion
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;
}

?>