PHP Trickery

While studying for my Zend certification, I came across a practice test question that asks what the primary difference is between a static method and a normal method. I figured this would be an easy question but I was almost tricked with false promises of language structure consistency*!

I was able to narrow my choices down to two answers: 1) static methods can’t be called from an instance and can only be called using the :: syntax, or 2) $this is not provided by static methods. I decided to play around with PHP a bit to find the answer and it turns out that the correct answer is number 2, which I knew but number 1 was throwing me off.

It turns out that you can call a static method from an instance, why you would want to I’m not sure.

Here’s an example:

class A {
    public static function foo() {
        return 'Hello';
    }
    public function bar() {
        return $this->foo();
    }
}
// normally I would just call:
echo A::foo();
// but this also works:
$instance = new A();
echo $instance->foo();
// and even this works!
$instance2 = new A();
echo $instance2->bar();

All of the examples above produce Hello.

Update: I will point out that this functionality is documented under Static Keyword in the PHP manual. Also, yesmarklapointe has a nice breakdown on what is returned under certain scenarios.

* PHP and Java have a lot of similarities when it comes to language structure but I guess most modern day languages do. I believe Java treats static methods the way you would expect–the only way to access a static method from outside of a Java class is by using the class name followed by the static method (e.g. A.foo()). To access foo() from within the same class you would just call foo().