I have found the answer.

The problem lies that the new version of firebase/php-jwt requires three arguments for the JWT::encode() method, while i am just putting two arguments there, therefor the token was not returned properly.

Originial JWT::encode

return JWT::encode($payload, self::$secret_key);

Update JWT::encode

return JWT::encode($payload, self::$secret_key, 'HS256');

Thank you for you time