It seems that when you wish to export a varible, you can do it as return $varible, return an array(), or globalise it. If you return something, information for that varible can only travel one way when the script is running, and that is out of the function. 

function fn() {
  $varible = "something";
  return $variable;
}

echo fn();
OR
$newvariable = fn();

Although if global was used, it creates a pointer to a varible, whether it existed or not, and makes whatever is globalised in the function linked to that global pointer. So if the pointer was global $varible, and then you set a value to $varible, it would then be accessible in the global scope. But what if later on in the script that global was redefine? This means that whatever is put into the global array, the information that is set in the pointer, can be set at any point (overiden). Here is an example that might make this a little clearer:

function fn1() {

   global $varible; // Pointer to the global array
   $varible = "something";
}

fn1();
echo $varible; // Prints something
$varible = "12345";
echo $varible; // Prints 12345

function fn2() {

   global $varible; // Pointer to the global array
   echo $varible;
}

fn2(); // echos $varible which contains "12345"

Basically with the global array, it can be set to refer to something already defined or set it to something, (a pointer) such as varible you plan to create in the function, and later possibly over ride the pointer with something else.

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This was originally posted in the manual, but was removed. I was notified of this via email, which suggested I do a bug report, or report it as a security flaw. I'm not sure which is best, so I entered in a bug report.