如何在php中的类内部引用静态函数？

Question

I'am trying to make a reference to a static function inside a class:

class Test {

    function __construct() {

        $this->fn1 = self::fn2;


    }

    public static function fn2() {

    }


}

then i get this error:

Undefined class constant 'fn2'

why?

Answer 1

You have defined a static function :

Test {
    function__construct()
    {
        $this->fn1 = self::fn2();
    }

    public static function fn2()
    {

    }
}

Answer 2

Updated

If you want to assign a function to a variable, it is best to do this with annonymous aka lambda functions since they are first class citizens and may be freely passed, returned and assigned. PHP is not unique in dealing with static method references in this fashion as JAVA implements them similarly:

Method references ... are compact, easy-to-read lambda expressions for methods that already have a name.

You may create an anonymous function based on a callable in PHP, and so the OP may wish to do as follows, which PHP 7.1.10 or higher supports:

<?php
class Test {
    public static function fn2() {
         return __METHOD__;
    }
    public static function getClosure (){
        return Closure::fromCallable(["Test","fn2"]);
    }
}


echo Test::getClosure()(),"\n";

See live code here

In this example an anonymous function is created and returned by the static getClosure method. When one invokes this method, then it returns the closure whose content is the same as static method fn2. Next, the returned closure gets invoked which causes the name of static method fn2 to display.

For more info re closures from callables, see the Manual and the RFC .

With PHP 7 on up, you may create a complex callable . In the code below the complex callable is an invocable array:

<?php
class foo 
{
    public static function test()
    {   
        return [__CLASS__, 'fn2'];
    }   

    public static function fn2()
    {   
        echo __METHOD__;
    }   
}

echo foo::test()();

See live code .

Note: Starting with PHP 7.0.23 you could create a complex callable using a string containing the class and method names separated by the double colon aka paaamayim nekudotayim; see here .

A solution that has broader PHP support is as follows:

<?php

class Test {

    public static function fn2() {
           return __METHOD__;
    }
    public static function tryme(){
      return call_user_func(["Test","fn2"]);
    }

}
// return closure and execute it
echo Test::tryme();

See live code

Answer 3

Not sure if this is what you want, but at least this might give you a hint:

<?php

class Test {

    function __construct() {

        $this->fn = function(){
             return self::realFn();
        };

    }

    public function callFn (){
        $fn = $this->fn ;//yes, assigning to a var first is needed. You could also use call_user_func
        $fn();
    }
    public static function realFn() {
        echo 'blah';
    }


}

$x = new Test();

$x->callFn();

You can test it here: https://3v4l.org/KVohi