Undefined constant token Error

I am seeing the following error with the plugin using PHP version 7.2.19 and WordPress version 5.2.2.

Use of undefined constant token – assumed ‘token’ (this will throw an Error in a future version of PHP) in <snip>/wp-content/plugins/poptin/poptin.php on line 209

I looked into the code and found this line was causing the error:

$curl_post_array = array(
token => $token,
user_id => $user_id
);

I changed the line to:

$curl_post_array = array(
‘token’ => $token,
‘user_id’ => $user_id
);

Adding the single quotes around “token” and “user_id” cleared the error.