title: WARNING: PACK(): IN xx ON LINE xxx
body: <p>I am getting a warning from the pack function without any information. The string I am passing seems to be fine. Can anyone tell me what might be causing the issue?</p>
  
  <pre><code>$s = trim(hash_hmac('sha1', trim($stringToSign), $key));
  $binary_hmac = pack(“H40”, $s);
  </code></pre>
  
  <p>$binary_hmac returns false.</p>