Question

Anybody knows the technical reason why this constraint is placed on PHP classes (at least in v5.1x)?

Answer 1

Constants cannot contain mutable types. A constant is a “variable” that cannot be changed; it cannot be assigned to, but if its value were mutable, then it could be changed just by mutating the value:

class SomeClass
{
    public const $array = array(0 => 'foo', 1 => 'bar');

    public static function someFunction()
    {
        self::$array[0] = 'baz'; // SomeClass::$array has now changed.
    }
}