Yes, that's not how global variables work in PHP.
You can do this to be certain:
$myVar = 16;
But generally speaking, you don't have to explicitly define that you're using a global unless you're inside a function. Functions get their own local variables, they have to declare a variable to be global to access the global variable space instead of their local variable space.
Read this, it's useful info to know:
Also note that you're not *declaring* a variable. PHP does not require, nor have, variable declaration. It creates variables when you reference them. The global keyword just says that references to the variable given are references to that variable in the global scope, not in a local scope.