Variables

Any variable used inside a function is by default limited to the local function scope. If you want to access a variable used outside of the function you need to declare it as global within the function:

function my_function ()
{
  global $the_outside_variable;
		
  echo $the_outside_variable;		//<<This now works!
}

Defines

No need to declare a define as global in functions, any function can simply use it

define("USER_MIN_AGE",             16);