I found a lot of solutions, but they all require a lot of libraries and dependencies.

I build my own solution with no additional dependencies. This is the api for getting OAuth2 tokens: https://developers.google.com/identity/protocols/oauth2/service-account#httprest

The first step is to create a JWT (Json Web Token). Using this JWT, a bearer token can be requested.

// php doesn't have build-in support for base64UrlEncoded strings, so I added one myself
function base64UrlEncode($text)
{
    return str_replace(
        ['+', '/', '='],
        ['-', '_', ''],
        base64_encode($text)
    );
}

// Read service account details
$authConfigString = file_get_contents("path_to_the_json_file_jou_downloaded_from_firebase_console.json");

// Parse service account details
$authConfig = json_decode($authConfigString);

// Read private key from service account details
$secret = openssl_get_privatekey($authConfig->private_key);

// Create the token header
$header = json_encode([
    'typ' => 'JWT',
    'alg' => 'RS256'
]);

// Get seconds since 1 January 1970
$time = time();

// Allow 1 minute time deviation
$start = $time - 60;
$end = $time + 3600;

$payload = json_encode([
    "iss" => $authConfig->client_email,
    "scope" => "https://www.googleapis.com/auth/firebase.messaging",
    "aud" => "https://oauth2.googleapis.com/token",
    "exp" => $end,
    "iat" => $start
]);

// Encode Header
$base64UrlHeader = base64UrlEncode($header);

// Encode Payload
$base64UrlPayload = base64UrlEncode($payload);

// Create Signature Hash
$result = openssl_sign($base64UrlHeader . "." . $base64UrlPayload, $signature, $secret, OPENSSL_ALGO_SHA256);

// Encode Signature to Base64Url String
$base64UrlSignature = base64UrlEncode($signature);

// Create JWT
$jwt = $base64UrlHeader . "." . $base64UrlPayload . "." . $base64UrlSignature;

//-----Request token------
$options = array('http' => array(
    'method'  => 'POST',
    'content' => 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion='.$jwt,
    'header'  =>
        "Content-Type: application/x-www-form-urlencoded"
));
$context  = stream_context_create($options);
$responseText = file_get_contents("https://oauth2.googleapis.com/token", false, $context);

$response = json_decode($responseText);

$response Contains the bearer token. You should store this token for use by other requests and request a new bearer token when it is about to expire. The maximum lifetime of this bearer token is 1 hour.