类 - 变量声明

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

You cannot use statement or function, just a scalar value. This is because class variables are initiated in compile time (before runtime). Class constructor should be used to initiate property with statement/function.