How would you declare a function and specify a default value for an attribute?

How would you declare a function and specify a default value for an attribute?

function printNum$($$num$)\{$

$num $=2$;

echo $num;

function printNum$($$num $=2)\{$

echo $num;

$\}$

You cannot set default variables.

function printNum$($$num$[2])\{$

echo $num;