A PHP function to sign dynamic parameters for FormAssembly

FA_Signing.php

<?php
  function prepareFormAssemblyQueryString(array $parameters, $salt, $expire_timeout) {
    $parts = [];
    $signString = '';
    foreach($parameters as $key => $value) {
      $signString .= $key . $value;
      $parts[] = rawurlencode($key) . '=' . rawurlencode($value);
    }
    $expireTime = time() + $expire_timeout;
    $signString .= 'expire' . (string)$expireTime;

    // hmach the signature string
    $hash = hash_hmac('sha256', $signString, $salt, TRUE);
    $sig = rawurlencode(base64_encode($hash));

    $parts[] = 'signature=' . $sig;
    $parts[] = 'expire=' . (string)$expireTime;
    
    $queryString = implode('&', $parts);

    return $queryString;
  }