1.php

<?php
class Foo
{
    public function bar()
    {
        // ...
        $posts = get_posts($condition);
        if ( !is_array($posts) ){
            return;
        }
        // ...
    }
}

2.php

<?php
class FooProxy
{
    public function __call($name, $args)
    {
        return call_user_func_array($name, $args);
    }
}

class Foo
{
    private $proxy;

    public function __construct(FooProxy $proxy)
    {
        $this->proxy = $proxy;
    }

    public function bar()
    {
        // ...
        $posts = $this->proxy->get_posts($condition);
        if ( !is_array($posts) ){
            return;
        }
        // ...
    }
}

3.php

<?php
class FooTest extends PHPUnit_Framework_TestCase
{
    /** @test */
    public function barReturnsNullWhenNoPosts()
    {
        $proxy = $this->getMockBuilder('FooProxy')
            ->setMethods(array('get_posts'))
            ->getMock();
        $proxy->expects($this->once())
            ->method('get_posts')
            ->will($this->returnValue(''));

        $foo = new Fooo($proxy);
        $this->assertNull($foo->bar());
    }
}