Question

Why I can’t call a constant from class B via dynamic property of class A like this? Am I doing something wrong?

class A {
    public $class_b;
}

class B {
    const CONST_VAR = 'b';
}

$class_a = new A();
$class_a->class_b = new B();

echo $class_a->class_b::CONST_VAR;

PHP Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM, expecting ‘,’ or ‘;’ in /root/1.php on line 14

However calling it like this is fine:

$b = $class_a->class_b;
echo $b::CONST_VAR;

Answer 1

Class constants are static. You have an instance of class B in the $class_b variable so you shouldn’t access it through class A unless you make a non-static function in class B that returns the constant. For example:

class B {
    const CONST_VAR = 'b';

    function get_constant() {
        return self::CONST_VAR;
    }
}

Now you can use:

 $class_a = new A();
 $class_a->class_b = new B();
 $class_a->class_b->get_constant();

But there is really no reason to do it this way unless you plan on overloading class b because you can just use B::CONST_VAR;

 echo B::CONST_VAR; // prints 'b'

Read about class constants here: http://php.net/manual/en/language.oop5.constants.php

It should be noted that as of php 5.3.0 constants can be accessed through instances such as you suggested, $b::CONST_VAR, but this is not how constants should be used and was most likely only added to support bad programming.