无法从数组中的类访问静态成员变量

Question

I am trying to access a static class member variable in an array.

My Code (index.php):

<?php

class Foo
{
    public static $staticVar = 'test';
}

class Bar
{
    public $someArray = array(
        Foo::$staticVar
    );
}

$cls = new Bar();

var_dump($cls->someArray);

?>

On PHP-7.0 I get this error:

PHP Fatal error: Constant expression contains invalid operations in /var/www/html/index.php on line 12

On PHP-5.6 I get this error:

PHP Parse error: syntax error, unexpected '$staticVar' (T_VARIABLE), expecting identifier (T_STRING) or class (T_CLASS) in /var/www/html/index.php on line 11

I just want to have the string "test" in my array.

It´s strange that when I 'echo' out the variable it works as expected:

echo Foo::$staticVar // prints 'test'

I´m new to PHP and I can´t figure out what I´m doing wrong.

Answer 1

Unfortunately, you can't refer to another variable or class in the initial declaration of a class property. It's just a limitation of the language as it stands. The general workaround is to initialise the property in the constructor, e.g.

class Bar
{
    public $someArray = array();

    public function __construct()
    {
        $this->someArray = array(
            Foo::$staticVar
        );
    }
}

On a vaguely related note, PHP 5.6 did at least make some vague headway in allowing you to define constants as basic expressions, see https://3v4l.org/6TDZV