API Callback X Signature Validation

<?php

function deep_map_pairs($data = array()){
  $a = array();

  foreach ($data as $k => $v){
    if ($k == 'x_signature'){
      continue;
    }
    if (is_array($v) && array_keys($v) !== range(0, count($v) - 1)) {
      $b = array();
      $dmp = deep_map_pairs($v);
      $flatted_dmp = new RecursiveIteratorIterator(new RecursiveArrayIterator($dmp));
      foreach ($flatted_dmp as $p){
        $b[] = $k.$p;
      }
      $a[] = $b;
    } else if (is_array($v)){
      $b = array();
      foreach ($v as $c){
         $b[] = deep_map_pairs(array($k => $c));
      }
      $a[] = $b;
    } else {
      $a[] = $k.$v;
    }
  }

  return $a;
}

$new = deep_map_pairs($_POST);
$flatted_new = new RecursiveIteratorIterator(new RecursiveArrayIterator($new));

$array_flatted = [];
foreach ($flatted_new as $p){
  $array_flatted[] = $p;
}

$compacted_af = array_filter($array_flatted, function($var){
   return ($var !== NULL && $var !== FALSE && $var !== "");
});

sort($compacted_af, SORT_REGULAR | SORT_FLAG_CASE);
$source_string = implode('|', $compacted_af);

$xsignature_key = 'S-R5t3Uw6SrwXNWyZV-naVHg'; // Get from your Billplz Account Settings
$xsignature = $_POST['x_signature'];
$equal = hash_equals(hash_hmac('sha256', $source_string, $xsignature_key), $xsignature);

if ($equal) {
    error_log('match');
} else {
    error_log('not match');
}