Variables within functions, out?

Hey guys. Can I firstly say, thanks to all of you who helped me out with my last question about importing MS Access databases into MySQL... It helped tons! However, I have another question. Firstly, I've strayed away from writing many of my own functions, because they give me the fear. But the code on the pages I am creating has forced me to wake up... I really sould think about cutting back on code. SO, anyway, to cut to the chase, my question is this: If I write a function, which performs checks and assigns variable based on these checks, how do I get the assigned variables from within the function? For example (this is more than likely VERY wrong), something like this: global.inc: <? function CheckBirthday() { global $year,$month,$day; if (($day) && ($month) && ($year)) { if (checkdate($day,$month,$year) { $birthday = "$year/$month/$day"; } else { $birthday = "Invalid"; } } } ?> File that would update / insert information to a MySQL database: <? // Connection details here require("global.inc"); CheckBirthday(); $sql = "UPDATE MyTable SET birthday = \"$birthday\" WHERE id = \"$id\" "; // etc ?> Even when I KNOW that I have included correct values, the $birthday variable never shows up outside the function. It's probably something simple I'm missing, as in most cases ;) Thanks for your patience, James.