PHP abstract properties

Question

Is there any way to define abstract class properties in PHP?

abstract class Foo_Abstract {
    abstract public $tablename;
}

class Foo extends Foo_Abstract          


        

Answer 1

I've asked myself the same question today, and I'd like to add my two cents.

The reason we would like abstract properties is to make sure that subclasses define them and throw exceptions when they don't. In my specific case, I needed something that could work with statically.

Ideally I would like something like this:

abstract class A {
    abstract protected static $prop;
}

class B extends A {
    protected static $prop = 'B prop'; // $prop defined, B loads successfully
}

class C extends A {
    // throws an exception when loading C for the first time because $prop
    // is not defined.
}

I ended up with this implementation

abstract class A
{
    // no $prop definition in A!

    public static final function getProp()
    {
        return static::$prop;
    }
}

class B extends A
{
    protected static $prop = 'B prop';
}

class C extends A
{
}

As you can see, in A I don't define $prop, but I use it in a static getter. Therefore, the following code works

B::getProp();
// => 'B prop'

$b = new B();
$b->getProp();
// => 'B prop'

In C, on the other hand, I don't define $prop, so I get exceptions:

C::getProp();
// => Exception!

$c = new C();
$c->getProp();
// => Exception!

I must call the getProp() method to get the exception and I can't get it on class loading, but it is quite close to the desired behavior, at least in my case.

I define getProp() as final to avoid that some smart guy (aka myself in 6 months) is tempted to do

class D extends A {
    public static function getProp() {
        // really smart
    }
}

D::getProp();
// => no exception...