OAuth2 returns invalid_client error

The problem is in the cURL headers you're setting something.php. Remove the Content-Type header. In fact, you can not set the headers at all - cURL will send the correctly encoded parameters and Box will return JSON data by default.

curl_setopt($ch, CURLOPT_HTTPHEADER, array(    'Accept: application/json'));

Here is how I received the token in JS

authorizeUser = function(){            var results = $.ajax({            // The URL to process the request            url : 'https://www.box.com/api/oauth2/token',            type : 'POST',            data : {                grant_type : 'authorization_code',                code : data.boxAuthorizationCode,                client_id : data.clientId,                client_secret : data.clientSecret            },            beforeSend: function (xhr) {  xhr.setRequestHeader("Authorization", "Bearer $token")},            dataType: "json",            success: function(response) {               //console.log(response);               console.log(response.access_token);               data.access_token = response.access_token;               tokenGranted();            }        });        return results.responseText;    },