PHPUnit testing file_get_contents

I have a class which has a method that makes use of PHP's global file_get_contents function. I need to test the method on the class, without actually calling the global function.

I know that I could use namespaces to override what gets returned from file_get_contents, however my tests are in a separate namespace already so I cannot simply match namespaces with the class.

Here's some code:

The class

<?php
namespace MyVendor\MyProject;

class MyClass
{

public function myMethod()
{
    $request = 'http://domain.com';
    $response = $this->someMethodUsingGlobals($request);
    // Do something with the response..
}
private function someMethodUsingGlobals($url)
{
    return json_decode(file_get_contents($url),true)['results'][0];
}

}

The test

<?php
namespace MyProjectTests;

public function test_it_does_something_with_the_response()
{
    $sut = new MyClass();

    $response = $sut->myMethod();

    $this->assertEquals('Some expectation', $response);
}

I need to mock the someMethodUsingGlobals() method on the class, but not entirely sure how to go about this.