Why don't PHP attributes allow functions?

I'm pretty new to PHP, but I've been programming in similar languages for years. I was flummoxed by the following:

class Foo {
    public $path = array(
        realpath(".")
    );
}

It produced a syntax error: Parse error: syntax error, unexpected '(', expecting ')' in test.php on line 5 which is the realpath call.

But this works fine:

$path = array(
    realpath(".")
);

After banging my head against this for a while, I was told you can't call functions in an attribute default; you have to do it in __construct. My question is: why?! Is this a "feature" or sloppy implementation? What's the rationale?