Twitter API token request returning gobbledygook

I'm trying to use Application Only Authentication, as described here:
https://developer.twitter.com/en/docs/basics/authentication/overview/application-only
I'm using the following PHP code to do so.

if(empty($_COOKIE['twitter_auth'])) {

        require '../../social_audit_config/twitter_config.php';

        $encoded_key = urlencode($api_key);
        $encoded_secret = urlencode($api_secret);
        $credentials = $encoded_key.":".$encoded_secret;
        $encoded_credentials = base64_encode($credentials);

        $request_headers = array(
            'Host: api.twitter.com',
            'User-Agent: BF Sharing Report',
            'Authorization: Basic '.$encoded_credentials,
            'Content-Type: application/x-www-form-urlencoded;charset=UTF-8',
            'Content-Length: 29',
            'Accept-Encoding: gzip'
        );

        print_r($request_headers);

        $ch = curl_init();
        curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
        curl_setopt($ch, CURLOPT_HTTPHEADER, $request_headers);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_URL, 'https://api.twitter.com/oauth2/token');  
        curl_setopt($ch, CURLOPT_POSTFIELDS, 'grant_type=client_credentials');
        $attempt_auth = curl_exec($ch);

        print_r($attempt_auth);

    }

It should return JSON with the token in it, but instead it returns gobbledygook, as seen in the image below:

I'm sure I'm missing some very simple step, where am I going wrong?
If I send the curl request without the headers, it returns an error in JSON format as expected, so is there something wrong with my headers?


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