[califield]

`f` in your example is a global function that cannot be redefined.

[imaginenore]

Don't like global functions? Don't use them.

It's really that simple.

[xyby]

Which is a good thing. When you have ...

    function f($x) { return $x*$x; }

...in your code, you know that you can always call f() to get the square of something. Hallelujah. Functions as they are meant to be.