Using Static Class Variables to Access Globally

The typical way to access a variable or instance from inside a function/method is to either declare it a global variable or pass it as a argument. Is there any reason why someone shouldn't use static class variables to do this? Ex: <?php class Foo { public static $bar_instance; } class Bar { public function do_something() {} } Foo::$bar_instance = new Bar; function foo_bar() { Foo::$bar_instance->do_something(); } foo_bar(); ?> Crude example but imagine this on a larger scale. I'm thinking there may be some kind of php optimization that this would hamper or something to that effect.