crypt52.php

<?php

$encoded = ['x/y', 'x/y'];
$chars = [];

// Calculate the full prime number
$prime_num = gmp_sub(gmp_pow($exp, $base), 1);
$prime = gmp_strval($prime_num);

// Convert encoded message to 5-bit characters
foreach ($encoded as $word) {
  list($offset, $length) = explode('/', $word);
  $number = substr($prime, $offset, $length);
  $number_bin = decbin($number);
  $number_pad = pad_to_nearest_five($number_bin);
  $number_array = str_split($number_pad, 5);
  $chars = array_merge($chars, $number_array);

}

$legend = ['', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
           'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', ' ',
           '.'];

// Convert 5-bit characters to decoded message
foreach ($chars as $key => $char) {
  echo $legend[bindec($char)];

  if (($key+1) % 80 == 0) {
    echo PHP_EOL;
  }
}

echo PHP_EOL;

// Given a string, pad it with zeros so that the total string length is
// divisible by five
function pad_to_nearest_five($number) {
  $length = strlen($number);

  while ($length % 5 != 0) {
    $length++;
  }

  return str_pad($number, $length, '0', STR_PAD_LEFT);
}