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up vote 1 down vote favorite

I understand that if you declare a variable within a php function with the 'global' keyword it will reference a variable declared outside the function, but why would a php programmer want to declare a variable outside of a function scope as 'global?' Thanks!

I understand what this does:

<?
$a = 1;
function $boo() {
    global $a;
    echo $a;
}
?>

But what I'm getting at is why would I want to do this?

<?
global $a;
function $boo() {
    //foo
}
?>
share|improve this question

3 Answers 3

up vote 2 down vote accepted

It has to do with php scope If you have file a.php that has a class like this

<?
class test()
{
  function test()
  {
    include('b.php');
  }
}
?>

and a file b.php

<?
$a = 1;
?>

Then $a will only be accessible in the scope of function test()

if you have global $a in b.php, $a then becomes a global variable

here's the php doc about it : http://php.net/manual/en/function.include.php

share|improve this answer
    
Good point. Although that is basically the same situation as the asker originally presented. All includes/requires are executed within the scope the include statement is in, so you're still within a function, and using it to break out of the function scope. The include just makes it less transparent what's going on. –  kander Jan 11 '11 at 18:41
    
Yes but that is the only reason why you would see a global keyword in the main scope of a php file (ie it's included elsewhere and hence need the global to break out of the scope). –  Damp Jan 11 '11 at 18:46
    
^This may be what's happening somewhere in the rest of this code. I do agree that this is can be unclear and confusing. –  Casey Flynn Jan 11 '11 at 18:46
    
Good explanation. So don't do globals people, it's up there with goto's ;-) –  Ward Bekker Jan 11 '11 at 20:22
up vote 0 down vote

I have no idea why you would want to do that. For all intents and purposes (unless I am very, very much mistaken) this is exactly the same as just:

<?
var $a;
function $boo() {
  // foo
}
?>

Which in turn is the very same as 

<?
function $boo() {
  // foo
}
?>

because you generally don't have to instantiate your variables in PHP.

Very curious why you're using variably-named functions though? (function $boo() {} )

share|improve this answer
    
Yeah I'm analyzing someone else's code and I'm trying to understand what's going on. I found this and I couldn't understand why it was being done. –  Casey Flynn Jan 11 '11 at 18:35
    
Wow... in that case... good luck! If they do this kind of thing, you're going to need it. (legacy codebases.. the joy, the suffering!) –  kander Jan 11 '11 at 18:36
up vote 0 down vote

Well, IMHO the use of global variables is a poor programming practice. It can cause unintended side-effects in your program which are hard to debug, and makes it harder to maintain.

share|improve this answer
    
While a very valid observation, this does not answer the question. –  kander Jan 11 '11 at 18:36
    
You are right. The PHP parser should throw a syntax error in this case, but it doesn't (surprise, surprise). –  Ward Bekker Jan 11 '11 at 18:39

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