How do I check in PHP that I'm in a static context (or not)?

Try the following:

class Foo {
   function bar() {
      $static = !(isset($this) && get_class($this) == __CLASS__);
   }
}

Source: seancoates.com via Google

"Digging it out of debug_backtrace()" isn't too much work. debug_backtrace() had a 'type' member that is '::' if a call is static, and '->' if it is not. So:

class MyClass {
    public function doStuff() {
        if (self::_isStatic()) {
            // Do it in static context
        } else {
            // Do it in object context
        }
    }

    // This method needs to be declared static because it may be called
    // in static context.
    private static function _isStatic() {
        $backtrace = debug_backtrace();

        // The 0th call is to _isStatic(), so we need to check the next
        // call down the stack.
        return $backtrace[1]['type'] == '::';
    }
}

Checking if $this is set won't work always.

If you call a static method from within an object, then $this will be set as the callers context. If you really want the information, I think you'll have to dig it out of debug_backtrace. But why would you need that in the first place? Chances are you could change the structure of your code in a way so that you don't.