Uninitialized variable 'x' or missing 'global' to refer to global variable

Sample code:

$x = 4; // global variable $x
//...
function fun() {
    $y = $x + 4;                   // Uninitialized variable 'x' or missing 'global' to refer to global variable
}

Codenizer detected attempt to use uninitialized variable within function with same name like global variable.