Why is global $object_name discouraged?

Question

I've got two classes, and currently I reference one from the other by using this:

ClassB::func()
{
    global $classAObject;
    echo $classAObject->whatever();
}

However, I've been told that using global is discouraged. Is it, and why?

http://softwareengineeringexplored.blogspot.com/2009/06/introduction-global-variables-are-those.html — Erik, May 02 '11 at 16:33
possible duplicate of [\[PHP\] global in functions](http://stackoverflow.com/questions/5166087/php-global-in-functions) — Gordon, May 02 '11 at 17:17
Thanks @Gordon - I'll have a read of that thread, as well as @Erik's link - they both look like good resources :-) — Bojangles, May 03 '11 at 13:03
Possible duplicate of [Are global variables in PHP considered bad practice? If so, why?](http://stackoverflow.com/q/1557787/1255289) — miken32, Mar 11 '17 at 18:09

Erik · Accepted Answer · 2011-05-02T16:59:31.850

There are many reasons not to use globals. Here's just a few:

Scope
- In a large system it can get easy to accidentally reassign global variables if you reuse a semi-common name
- Variables in Global Scope increase your scripts memory footprint. Not always important, but can be
- In some other languages, its not necessary to grab your global variables - they are available by default - this can lead to bugs if you forget to declare a same-named variable as local
Coupling
- In good software design, components should be loosely coupled. Global variables implies tight coupling.
Maintenance
- A global variable can be changed by any other code, anywhere. Especially in large systems this can make debugging existing code or adding new code a nightmare.

A better way to handle the example you gave in your post would be to pass the object containing the data you need.

classB::func($obj) 
{
   echo $obj->whatever();
}

$obj = new classAObject;
classB::func($obj);

Thank you for your better solution and the points above - very helpful answer! I'd upvote this like 5 times but I can't ;-) — Bojangles, May 03 '11 at 13:01

score 0 · Answer 2 · edited May 02 '11 at 16:45

0

The reason is that it flubs the idea of OOP encapsulation. It's much better to do:

ClassB::func($classAObject)
{
    echo $classAObject->whatever();
}

edited May 02 '11 at 16:45

answered May 02 '11 at 16:30

Naftali

2 Answers2