• many forget global variables are fully global, not only to the current file;
• declaration outside any other scope in the file doesn't necessarily mean the variable will be global, e.g. if the current file is included from a non-root scope. The only completely reliable way to declare a global is to use $GLOBALS;
• fully global variables are generally a bad idea anyway.

Instead, if you need a similar feature, look at class (or function) static variables.

• they have the expected scope;
• support access restriction (private, protected, public);
• can be referenced from the using class with the better syntax self::$my_class_global.

If you don't have a class… well, first check if your code couldn't get better organized by being in one. If not, either create a state class (abstract class MyState { static $foo = 0; }) and use it as a global, or know what you do and use a global, with all its attached baggage.