Mocking Method with Random Input Using shouldReceive in PHPUnit

I am using Laravel 4 with PHPUnit 3.7.38. I have a class that uses a randomly generated value for input. I use str_random(100). I want to unit test it; however, everytime, I try to use shouldReceive on the method, I get an error saying that the value does not match the expected value. This makes sense since it randomly generates the value before testing it. I can not refactor the original code and I can not throw an exception halting the code because there is more to test after this code in the same function. I have tried mocking the str_random function; however, that throws another error. I'm really not sure what to do or if there is a good solution at this point in time. My code looks like:

class OriginalClass {

public function someFunction{

$randomCode = str_random(100);

    anotherFunction(array($name, $lastName, $randomCode), $anotherValue)

    }

}



class OriginalClassTest {



        $this->MockObject
            ->shouldReceive('anotherFunction')
            ->with(array('Jon', 'Smith', '11155488321'), '49')
            ->andReturn('Output');

        $this->assertEquals('Output', $this
            ->MockObject
            ->anotherFunction(array('Jon', 'Smith', '11155488321'), '49')); 
}

By the time the function is called a newly generated $randomCode has been created and I get the error

"Mockery\Exception\NoMatchingExpectationException : No matching handler found for Mockery_1_MockObject::anotherFunction(array('Jon', 'Smith', 'newly generated randomCode'), '49')"