Why don't PHP attributes allow functions?

Question

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?

Answer 1

My question is: why?! Is this a "feature" or sloppy implementation?

I'd say it's definitely a feature. A class definition is a code blueprint, and not supposed to execute code at the time of is definition. It would break the object's abstraction and encapsulation.

However, this is only my view. I can't say for sure what idea the developers had when defining this.

Answer 2

It's a sloppy parser implementation. I don't have the correct terminology to describe it (I think the term "beta reduction" fits in somehow...), but the PHP language parser is more complex and more complicated than it needs to be, and so all sorts of special-casing is required for different language constructs.

Answer 3

My guess would be that you won't be able to have a correct stack trace if the error does not occur on an executable line... Since there can't be any error with initializing values with constants, there's no problem with that, but function can throw exceptions/errors and need to be called within an executable line, and not a declarative one.

Answer 4

You can probably achieve something similar like this:

class Foo
{
    public $path = __DIR__;
}

IIRC __DIR__ needs php 5.3+, __FILE__ has been around longer