How can I get PHPUnit MockObjects to return different values based on a parameter?

Use a callback. e.g. (straight from PHPUnit documentation):

<?php
class StubTest extends PHPUnit_Framework_TestCase
{
    public function testReturnCallbackStub()
    {
        $stub = $this->getMock(
          'SomeClass', array('doSomething')
        );

        $stub->expects($this->any())
             ->method('doSomething')
             ->will($this->returnCallback('callback'));

        // $stub->doSomething() returns callback(...)
    }
}

function callback() {
    $args = func_get_args();
    // ...
}
?>

Do whatever processing you want in the callback() and return the result based on your $args as appropriate.

From the latest phpUnit docs: "Sometimes a stubbed method should return different values depending on a predefined list of arguments. You can use returnValueMap() to create a map that associates arguments with corresponding return values."

$mock->expects($this->any())
    ->method('getConfigValue')
    ->will(
        $this->returnValueMap(
            array(
                array('firstparam', 'secondparam', 'retval'),
                array('modes', 'foo', array('Array', 'of', 'modes'))
            )
        )
    );

I had a similar problem (although slightly different... I didn't need different return value based on arguments, but had to test to ensure 2 sets of arguments were being passed to the same function). I stumbled upon using something like this:

$mock = $this->getMock();
$mock->expects($this->at(0))
    ->method('foo')
    ->with(...)
    ->will($this->returnValue(...));

$mock->expects($this->at(1))
    ->method('foo')
    ->with(...)
    ->will($this->returnValue(...));

It's not perfect, since it requires that the order of the 2 calls to foo() is known, but in practice this probably isn't too bad.