Using PHP namespaces to mock functions

function_mock.php

<?php

// Define the function in the namespace that the caller is in.
namespace App {
    function query() {
        return call_user_func_array([$GLOBALS['__doubles']['query'], 'call'], func_get_args());
    }
}

namespace Tests {

    // Create a basic interface we can mock
    interface FunctionProxy { public function call(); }

    class ServiceTest extends \PHPUnit_Framework_TestCase
    {
        public function testSomething()
        {
            $mock = $this->prophesizeFunction('query');

            $mock->call('SELECT * FROM users')->shouldBeCalled()->willReturn('foo');

            $this->assertEquals('foo', (new \App\Service)->users());
        }

        private function prophesizeFunction($name)
        {
            $double = $this->prophesize(FunctionProxy::class);

            $GLOBALS['__doubles'][$name] = $double->reveal();

            return $double;
        }
    }
}