Add parity bits to a 7 byte DES encryption key

addParityBits.php

<?php

/**
 * @param string $key - Input 7 byte DES key, without parity bits, in hex format
 * @return string     - Output 8 byte DES key, with parity bits, in hex format
 */
function addParityBitsToDESKey($key) {
  $binary = base_convert($key, 16, 2);
  $bytes = str_split($binary, 7);

  foreach ($bytes as $key => $byte) {
    $ones = substr_count($byte, '1');
    $bytes[$key] .= ($ones % 2 == 0 ? '1' : '0');
  }

  $characters = array_map(function($byte) {
    return chr(base_convert($byte, 2, 10));
  }, $bytes);

  $string = implode('', $characters);

  return bin2hex($string);
}