Mocking function call with PHPUnit

<?php
namespace Test;

function file_get_contents($file) {
    return 'Foo Bar';
}

class MockingAFunctionCall extends \PHPUnit_Framework_TestCase
{
    public function testIWillCallAMockFunction()
    {
        $this->assertEquals(file_get_contents('fool.txt'), 'Foo Bar');
    }
}