Mock Object variable in PHPUnit

Question

I want to create a test for the following function in Cakephp2.0 with PHPUnit:

public function matching($check, $field) {

    $return = true;
    foreach ($check as $c) {
        if ($c != $this->data['User'][$field]) {
            $return = false;
        }         
    }

    return $return;
}

How do I set the value of:

$this->data['User'][$field]

My test function is:

public function testMatching() {
    $this->data['User']['password'] = 'testpass';
    $check = array('testpass');
    $result = $this->User->matching($check, 'password');
    $this->assertEquals(true, $result);
}

Thanks.