Undefined method on mock object implementing a given interface in PHPUnit?

It's because there is no declaration of "getClass" method in ConfigurationInterface. The only declaration in this interface is method "getAliasName".

All you need is to tell the mock what methods you will be stubing:

$cls = 'Sensio\Bundle\FrameworkExtraBundle\Configuration\ConfigurationInterface';
$mock = $this->getMock($cls, array('getClass', 'getAliasName'));

Notice that there is no "getClass" declaration but you can stub/mock non existing method as well. Therefor you can mock it:

$mock->expects($this->any())
    ->method('getClass')
    ->will($this->returnValue('Some\Other\Class'));

But in addtion you need to mock "getAliasName" method as well as long as it's interface's method or abstract one and it has to be "implemented". Eg.:

$mock->expects($this->any())
   ->method('getAliasName')
   ->will($this->returnValue('SomeValue'));

Cyprian's answer helped me, but there's a gotcha to be aware of. You can mock classes that don't exist, and PHPUnit won't complain. So you could do

$mock = $this->getMock('SomeClassThatDoesntExistOrIsMisspelledOrPerhapsYouForgotToRequire');

This means that if ConfigurationInterface doesn't exist at that point during runtime, you'll still get a message like

Fatal error: Call to undefined method Mock_ConfigurationInterface_21e9dccf::getClass().

If you're sure the method really exists on the class, then the likely problem is the class itself doesn't exist (because you haven't required it, or you misspelled it, etc).

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The OP is using an interface. Be advised that you must call getMock without specifying the list of methods to override, or if you do, you must either pass array(), or pass ALL the method names, or you'll get an error like the following:

PHP Fatal error: Class Mock_HttpRequest_a7aa9ffd contains 4 abstract methods and must therefore be declared abstract or implement the remaining methods (HttpRequest::setOption, HttpRequest::execute, HttpRequest::getInfo, ...)

Tyler Collier's warning is fair but doesn't contain a code snippet on how to code around it. Note this is very nasty and you should fix the interface instead. With that warning added:

$methods = array_map(function (\ReflectionMethod $m) { return $m->getName();}, (new \ReflectionClass($interface))->getMethods());
$methods[] = $missing_method;
$mock = $this->getMock($interface,  $methods);