在PHP中使用静态成员是否有最佳实践？

Question

PHP allows use of static member functions and variables, since 5.3 including late static bindings:

class StaticClass {
    public static $staticVar;

    ...
}

$o = new StaticClass();

Currently, there are various options to access those static members:

$o->staticVar;  // as instance variable/ function
$o::staticVar;  // as class variable/ function

Other options exist for accessing members from inside the class:

self::$staticVar;   // explicitly showing static usage of variable/ function
static::$staticVar; // allowing late static binding

Restructuring some existing classes that make some use of static members I've asked myself if there are best practices for working with static members in PHP?

Answer 1

Well, obviously, they all do different things.

$o->staticVar

This is invalid, since you cannot/should not access static properties with the instance property syntax.

StaticClass::$staticVar

This very plainly accesses a specific static variable on a very specific class.

$o::$staticVar

This accesses the static variable on the class that $o is an instance of. It's mostly used as a shorthand for the previous method and is equivalent in all respects. Obviously though, which class is used exactly depends on what class $o is an instance of.

self::$staticVar

This can be used only inside a class, and will always refer to the class that it's written in. It's a good idea to use this inside a class instead of StaticClass::$staticVar if the class refers to itself, since you don't need to worry about anything if you change the class name later. E.g.:

class Foo {

    protected static $bar = 42;

    public function baz() {
        self::$bar;  // good
        Foo::$bar    // the same, but should be avoided because it repeats the class name
    }

}

static::$staticVar

This can also only be used inside a class and is basically the same as self above, but resolves with late static binding and may hence refer to a child class.

What the "best practice" is is debatable. I'd say you should always be as specific as necessary, but no more. $o::$staticVar and static::$staticVar both allow the class to vary through child classes, while self::$staticVar and StaticClass::$staticVar do not. Following the open/closed principle, it's a good idea to use the former, more variable method to allow for extensions.

Properties, both static and non-static, should also not be public to not break encapsulation.

Also see How Not To Kill Your Testability Using Statics.