Help me understand global $$obj;

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May 30th, 2004, 07:59 PM

Archemides

Registered User

Join Date: May 2004

Posts: 25

Time spent in forums: < 1 sec
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Help me understand global $$obj;

I am trying to learn OOP (object oriented programming) in PHP.

I am working on the following example which may be found at

http://aspn.activestate.com/ASPN/do...f.classobj.html

I am having great difficulty understanding the purpose of the line

global $$obj;

as it appears in function class_parentage .... (see below)

I understand that globals are variables with a greater scope, such that

other functions, or even other scripts possibly, may access them.

But where in the code below is $$obj initialized, unless it is initialized

somehow by is_subclass_of ?

When I google search for the syntax and definition of is_subclass_of
it is not clear that anything is done to the first parameter.

I have also looke here: http://www.zend.com/manual/language.variables.scope.php

I cannot see in these code examples where $$obj ever gets initialized.

I am also confused as to whether $$obj is some separate variable from

$obj or whether it is somehow a syntax with an extra $ sign which elevates $obj to a global

status.

It is really driving me crazy, when I am trying to understand and learn
something new, difficult, alien, and there is something in the midst of

it all (like this $$obj) which seems to work, and yet makes no sense

as to the whys and wherefores of its use.

To thicken this sauce a bit.... I did a google search on class_parentage
and found that this same example appears in various languages.

In the FRENCH version, I find:
http://www.nexen.net/docs/php/annot...bj.examples.php

function class_parentage($obj, $class) {
if (is_subclass_of($GLOBALS[$obj], $class)) {
echo "L'objet $obj appartient à la classe ".get_class($$obj);
echo " est une sous-classe de $class\n";
} else {
echo "L'objet $obj n'est pas une sous-classe de $class\n";
}
}

(as an aside, non-sequitur, I must say that THIS url looks like something I should come back to

for more study

http://www.weberdev.com/search.php3...s&secondary=PHP

)

Thanks for any enlightenment.

============================================

function class_parentage($obj, $class) {

global $$obj;

if (is_subclass_of($$obj, $class)) {

echo "Object $obj belongs to class ".get_class($$obj);

echo " a subclass of $class\n";

} else {

echo "Object $obj does not belong to a subclass of $class\n";

}

}

=========================== Complete tutorial example

save as classes.inc

<?php

// base class with member properties and methods

class Vegetable {

var $edible;

var $color;

function Vegetable( $edible, $color="green" ) {

$this->edible = $edible;

$this->color = $color;

}

function is_edible() {

return $this->edible;

}

function what_color() {

return $this->color;

}

} // end of class Vegetable

// extends the base class

class Spinach extends Vegetable {

var $cooked = false;

function Spinach() {

$this->Vegetable( true, "green" );

}

function cook_it() {

$this->cooked = true;

}

function is_cooked() {

return $this->cooked;

}

} // end of class Spinach

?>

=============================

save as test_script.php

<pre>

<?php

include "classes.inc";

// utility functions

function print_vars($obj) {

$arr = get_object_vars($obj);

while (list($prop, $val) = each($arr))

echo "\t$prop = $val\n";

}

function print_methods($obj) {

$arr = get_class_methods(get_class($obj));

foreach ($arr as $method)

echo "\tfunction $method()\n";

}

function class_parentage($obj, $class) {

global $$obj;

if (is_subclass_of($$obj, $class)) {

echo "Object $obj belongs to class ".get_class($$obj);

echo " a subclass of $class\n";

} else {

echo "Object $obj does not belong to a subclass of $class\n";

}

}

// instantiate 2 objects

$veggie = new Vegetable(true,"blue");

$leafy = new Spinach();

// print out information about objects

echo "veggie: CLASS ".get_class($veggie)."\n";

echo "leafy: CLASS ".get_class($leafy);

echo ", PARENT ".get_parent_class($leafy)."\n";

// show veggie properties

echo "\nveggie: Properties\n";

print_vars($veggie);

// and leafy methods

echo "\nleafy: Methods\n";

print_methods($leafy);

echo "\nParentage:\n";

class_parentage("leafy", "Spinach");

class_parentage("leafy", "Vegetable");

?>

</pre>

===================

OUTPUT when executed:

===================

veggie: CLASS vegetable

leafy: CLASS spinach, PARENT vegetable

veggie: Properties

edible = 1

color = blue

leafy: Methods

function vegetable()

function is_edible()

function what_color()

function spinach()

function cook_it()

function is_cooked()

Parentage: Object leafy does not belong to a subclass of Spinach

Object leafy belongs to class spinach a subclass of Vegetable

May 31st, 2004, 09:52 AM

Archemides

Registered User

Join Date: May 2004

Posts: 25

Time spent in forums: < 1 sec
Reputation Power: 0

Help came from IRC #phpfreaks

After I posted my question here, I did some google.com searching, and then decided to download mIRC irc client, and join the #phpfreaks channel.

If you go there, you must be patient and polite and persistant, (but not pesky), until someone feels like answering your question, but people there are quite knowledgable, and it is realtime response.

Visit this link for detail on downloading the mIRC chat client and joining #phpfreaks

http://www.phpfreaks.com/ircchan.php

A very knoweldgable person there explained to me that "$$" denotes a VARIABLE VARIABLE.

For example,
$x = "hello";

echo $x; prints the value of $x, which is "hello"

BUT

when I say

$$x = "Goodbye!" , what I am really doing is saying $hello = "goodbye"

$$x is a synonym for a variable called $hello.

So, if I write

echo $$x ; it will pring "goodbye";

but should I write $x = "bananna" ; why then, $$x will be a variable called $bananna.

I have not experimented with what happens to the variable $hello, possibly it still exists and can
be accessed.

Anyway, I am now much closer to understanding my OOP example, because $obj is passed to the function
as an argument, and when you write GLOBAL $$obj; then you are referencing some variable whose name is whatever the value of $obj is. So if $obj = "leafy" or "spinach", why then $$obj will reference some variable named $leafy or $spinach.

I know this seems very tricky and confusing, but with constant practice and repetition, I hope to make such things more familiar, and second nature.

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