Why it's not possible to use any magic constants in a static context?

Question

Is there any particular reason to deny the use of magic constants (like __DIR__) in PHP when in a static context? OOP POV seems nothing is wrong since __DIR__ is "global" that is not instance specific:

class MyClass
{

    protected static $options = array(
            'key1' => 2,
            'key2' => __DIR__ .'/mypic.png' // Triggers a syntax error
    );

    public function __construct() {}

}

EDIT clarification about the question. I'm interested in "why" it's not possible. For example, speaking about static stuff, anyone knows this is not possible:

public static function getName() { return $this->name; }

because getName() is shared across multiple instances and $this->name doesn't make any sense because static function "doesn't know" what instance to refer.

Answer 1

It isn't the use of __DIR__ or magic constants specifically here that isn't permissible. It is the dynamic construction of anything inside a property definition that isn't allowed. You'd also get a syntax error if you tried to concatenate two atomic strings like "a" . "b" inside a static property definition.

From the docs:

Like any other PHP static variable, static properties may only be initialized using a literal or constant; expressions are not allowed. So while you may initialize a static property to an integer or array (for instance), you may not initialize it to another variable, to a function return value, or to an object.... And concatenation, even onto a constant, counts as an expression which isn't allowed.

The PHP manual doesn't specify why initializing static variables with the result of an expression is a parse error, but it is likely due to the fact that static declarations are resolved at compile time, not runtime.