Scope can be a confusing concept for many beginners in PHP. The idea is that some variables and functions are invisible to each other in certain circumstances of code.

If you declare a variable outside of a function, let's say $C_array. The function cannot see it until you declare it as a global inside the function.

$C_array = array(5,2,1);
function Test(){
  global $C_array; // without this, C_array would be out of scope.
  $C_array[] = 7;
}

Alternatively you can use the superglobal $GLOBALS['C_array'].

Static Variables

Static variables can be useful for recursive functions.

Most variables declared inside a function are in the local scope and they stay there until the function finishes. However, if you restart the function recursively you may want a variable that doesn't just disappear when the function restarts.

function Recursive(){
  static $count = 0; // do not put an expression here.
  $count++;
  if($count < 25){
    Recursive();
  }
  count--;
}

Scope Operator ::

As long as you have a static function in the class, you can call a class's function without having to initiate the class. Just make sure you don't use a function that is pulling variables that were established in the constructor or the class that isn't static.