Ably SDK Encryption Issue

Question

I am using Ably, specifically Ably's PHP SDK to send a direct push notification via the web:

  $ably = new AblyRest(['key' => env('ABLY_KEY'));

  // Define the recipient using the subscription endpoint
  $recipient = [
    'transportType' => 'web',
    'endpoint' => $subscriptionEndpoint,
    'targetUrl' => 'https://yourdomain.com/push-notifications/subscribe',
    'encryptionKey' => ...,
  ];

For the encryption key, I am not sure exactly what I am supposed to put and what format it's meant to be. I have tried:

'encryptionKey' => \Ably\Utils\Crypto::generateRandomKey(256),

and

'encryptionKey' => bin2hex(\Ably\Utils\Crypto::generateRandomKey(256)),

and

'encryptionKey' => base64encode(\Ably\Utils\Crypto::generateRandomKey(256)),

But no matter what, I keep getting this error in my ably dashboard: "bad encryption key : [..encryption key..]"