Hey @haroon.momin,
What are you trying to accomplish? If you need to retrieve the user’s profile, you can call our /userinfo endpoint with an Access Token as a parameter.
Regarding how to decode the id_token, you can take a look at https://auth0.com/docs/libraries/auth0-php/validating-jwts-with-auth0-php or just visit https://jwt.io/ where you have a lot of libraries and information about the topic.