[SOLVED] Custom function in the event of a button

I am trying to create a function on a button (server side) but it is returning error.

Here is a simple example:

Button SUM

SERVER SIDE

$num01 = 10;

$num02 = 30;

$result["sum"] = calcSum($num01,$num02);

function calcSum($num01,$num02){

return ($num01 + $num02);
}


The error message is: "Call to undefined function calcSum() in ....\buttonhandler.php on line ..."