crypt28.php

<?php

$encoded = [
  ['55', '66', '78', '80', '00', 'FF', 'FF', '00'],
];

$decoded = '';
foreach ($encoded as $hex_word) {
  $bin_word = [];

  foreach ($hex_word as $hex) {
    $bin = str_pad(base_convert($hex, 16, 2), 8, '0', STR_PAD_LEFT);
    $bin_word[] = str_split($bin, 1);
  }

  for ($i = 7; $i >= 0; $i--) {
    $new_bin = '';
    for ($j = 7; $j >= 0; $j--) {
      $new_bin .= $bin_word[$j][$i];
    }
    echo chr(base_convert($new_bin, 2, 10));
  }
}