php - Class - variable declaration

Question

When the declaration of a PHP class variable we cannot perform any expressions, e.g.:

class A
{
    $a = 10 + 5;
}

only we can just provide constants e.g.:

class A
{
   $a = 100;
}

Can anybody knows why its like that?

Answer 1

That is because expression is not allowed as field default value. Make use of constructors to initialize the variables instead.

I suggest you do like this..

class A
{
    public $a;

    function __construct()
    {
        return $this->a = 10 + 5;
    }
}

$a1 = new A;
echo $a1->a; //"prints" 15