pack() in php. Illegal hex digit warning

Question

i am having some problems using pack() in php

$currencypair = "EUR/USD";
$buy_sell = "buy";
$alert_device_token =array("a","a","b");
$message = "Your " . $currencypair . " " . $buy_sell . " alert price has been reached!";
$payload['aps'] = array (
  'alert' => $message,
  'badge' => 1,
  'sound' => 'default'
);
$payload = json_encode($payload);

foreach ($alert_device_token as $alert_device)
{
  $apnsMessage = chr(0) . chr(0) . chr(32) . 
                 pack('H*', str_replace(' ', '', $alert_device)) . 
                 chr(0) . chr(strlen($payload)) . $payload;
  echo $apnsMessage;
}

Now sometimes i get following warnings running the same code -

Warning: pack() [function.pack]: Type H: illegal hex digit g in /code/FR2BVl

the illegal hex digit keeps varying though. Any ideas about this warning and ways to remove it.

check it live here

Answer 1

pack converts hexadecimal number to binary, e.g.:

  echo pack("H*", "2133")

produces !3, since ! has code 0x21 and 3 has code 0x33. Since g is not hex digit, warning is given. To be useful for pack's H format, the argument must be hex number. If $alert_device isn't - you should use something else, depending on what it is and what you expect as the result.

Answer 2

I was having the same issue when developing a hybrid app using Ionic/Cordova/PhoneGap. As the same code is run in Android and iOS devices, I had made a mistake of storing Google FCM token as APNS token. The APNS token is purely hexadecimal but Google FCM token can have non-hexadecimal characters. So, packing a Google FCM token using PHP's pack() function will result in the illegal hex digit error.