在PHP中定义只读抽象属性

Question

I want to do something like the following for an abstract class in PHP:

abstract class SimpleAbstractObject {
    // Not valid PHP code
    abstract protected static $Description;

    public static function getDescription(){
        return self::$Description;
    }
}

Naturally, the above fails to run:

Fatal error: Properties cannot be declared abstract

The idea is that any class extending the 'SimpleAbstractObject' has to define the "Description" variable and it can not be changed after it's defined. For example:

class SimpleObject extends SimpleAbstractObject {
    protected static $Description = "This is a Simple Object";
}
class AnotherObject extends SimpleAbstractObject {
    protected static $Description = "This is a Another more complicated object";
}

So that the end result is used something like this:

echo SimpleObject::getDescription();
echo AnotherObject::getDescription();

The above would print out:

This is a Simple Object
This is a Another more complicated object

Answer 1

You can use the static keyword for its late-static-binding properties since 5.3.

abstract class SimpleAbstractObject {
    public static function getDescription() {
        if(!isset(static::$Description))
            throw new UnexpectedValueException(
              get_called_class() .
              ' must define a static getDescription method');
        return static::$Description;
    }
}

The subclasses remain as you have them

class SimpleObject extends SimpleAbstractObject {
    protected static $Description = "This is a Simple Object";
}
class AnotherObject extends SimpleAbstractObject {
    protected static $Description = "This is a Another more complicated object";
}

but now work as expected

This is a Simple Object This is a Another more complicated object

The only disadvantage of this approach is you loose the 'compile time' check.