My static methods are either of the 'helper' variety, e.g.
In the case of a static class that is strictly a helper function like "convertToCamelCase" I would probably just have 100% coverage for that function and then consider it a "core" function and not worry about mocking it elsewhere. What is a mock for "convertToCamelCase" going to do anyway..? Perhaps your unit tests start to smell a little like integrations tests if you do it too much, but there's always a bit of a tradeoff between abstracting everything and making your app needlessly complicated.
As far as singletons it is tricky because you usually have the name of the static class in your code so it becomes problematic to swap it out with a mock object for testing. One thing you could do is wherever you are making your static method calls, start by refactoring to call them this way:
$instance = call_user_func('MyClass::getinstance');
Then, as you increase your testing coverage, you could begin replacing with something like:
$instance = call_user_func($this->myClassName . '::getinstance');
So - once you have that, you could swap out and mock
MyClass by changing
$this->myClassName. You'd have to make sure you're requiring or autoloading the relevant php files dynamically as well.
Refactoring to use an abstract factory pattern would make things even easier to test but you could start implementing that over time as well