file_get_contents返回NULL

Question

I am trying to fetch some data like first name, last name from LinkedIn. I am using the below code for the same. But I am getting NULL in response. I tried some solutions from SO and other websites but none of them worked.

Below is the code I am using:-

$url = 'http://api.linkedin.com/v1/people/~:(id,first-name,last-name,picture-urls::(original),headline,public-profile-url,location,industry,positions,email-address)?format=json&outh2_access_token='.$accessToken;
        $user = file_get_contents($url,false);
        var_dump(json_decode($user));
        die();

Below is the response that I am getting.

[16-Jul-2018 12:27:20 Europe/London] PHP Warning:  file_get_contents(http://api.linkedin.com/v1/people/~:(id,first-name,last-name,picture-urls::(original),headline,public-profile-url,location,industry,positions,email-address)?format=json&outh2_access_token=###########): failed to open stream: HTTP request failed! HTTP/1.0 401 Unauthorized

Basically what I want is to fetch the data from LinkedIn using php, if anybody has done this before please share the solution with me.