PHP function with variable as default value for a parameter

No, this isn't possible, as stated on the Function arguments manual page:

The default value must be a constant expression, not (for example) a variable, a class member or a function call.

Instead you could either simply pass in null as the default and update this within your function...

function actionOne($id=null) {
    $id = isset($id) ? $id : $_GET['ID'];
    ....
}

...or (better still), simply provide $_GET['ID'] as the argument value when you don't have a specific ID to pass in. (i.e.: Handle this outside the function.)

Well, why don't you just test ? ;-)

Note: It is not as easy as you'd think -- read the full answer ;-)

Calling the hello_testing(); function, without setting the variable:

hello_testing();

I get no output -- which indicates isset returned false.

Calling the function, after setting the variable:

$conditional_random = 'blah';
hello_testing();

I get an output:

foo is inside

Which indicates global works as expected, when the variable is set -- well, one should not have any doubt about that ^^

But note that isset will return false if a variable is set, and null!

See the manual page of isset()

Which means that a better test would be:

function hello_testing() {
    global $conditional_random;
    var_dump($conditional_random);
}

hello_testing();

And this displays:

null

No Notice: the variable exists! Even if null.

As I didn't set the variable outside of the function, it shows that global sets the variable -- but it doesn't put a value into it; which means it's null if not already set outside the function.

While:

function hello_testing() {
    //global $conditional_random;
    var_dump($conditional_random);
}

hello_testing();

Gives:

Notice: Undefined variable: conditional_random

It proves that notices are enabled ;-)

And, if global didn't "set" the variable, the previous example would have given the same notice.

And, finally:

function hello_testing() {
    global $conditional_random;
    var_dump($conditional_random);
}

$conditional_random = 'glop';
hello_testing();

Gives:

string 'glop' (length=4)

(This is to purely to demonstrate my example is not tricked ^^)

Use None as a default value:

def func(items=None):
    if items is None:
        items = []
    print items

The problem with a mutable default argument is that it will be shared between all invocations of the function -- see the "important warning" in the relevant section of the Python tutorial.