Client.php

<?php

class Client
{
    public function load($file) {
        return $this->getContents($file);
    }

    protected function getContents($file)
    {
        return file_get_contents($file);
    }
}

ClientTest.php

<?php

require __DIR__.'/../Client.php';

class ClientTest extends PHPUnit_Framework_TestCase
{       
    public function testLoad()
    {
        $client = $this->getClientMock();
    
        $this->assertEquals('Hello, yudoufu', $client->load('http://yudoufu.com/test'));
    }   

    protected function getClientMock()
    {
        $client = $this->getMock('Client', array('getContents'));

        $client->expects($this->any())
            ->method('getContents')
            ->will($this->returnValue('Hello, yudoufu'))
        ;

        return $client;
    }
}