help with code

Ok to break it down for you quickly...

<?
$CategoryID = $_GET['CategoryID'];

The $_GET function is used to collect values in a form where the method is "get", so in this case it's looking for a "CategoryID" value in a form that was submitted in the previous page. It's then assigning that value to a variable called $CategoryID (variables should begin in lowercase!!)

function get_param($param_name) {

Easy enough, declaring a function called get_param, and passes in a value $param_name

$param_value = "";

Declaring a variable to store the result, this will be returned at the end of the function.

if(isset($_POST[$param_name])) {
  $param_value = $_POST[$param_name];
}

An "if" block which checks if the parameter $param_name (value of whatever passed into the function) exists, and if so, assign that value to $param_value which will be eventually returned.

The built-in $_POST function is used to collect values from a form sent where the method is "post".

else if(isset($_GET[$param_name])) {
  $param_value = $_GET[$param_name];
}

If the parameter is not found using the $_POST function in the if block, we are trying to see if it exists for the $_GET function (explained at the top). If it is found, again assign the value of the parameter to the $param_value variable.

"post" and "get" are basically 2 different methods of passing data via a form, get sends the paramters in the URL. See here for more.

return $param_value;
}
?>

Finally, return the $param_value, if the parameter you are looking for was not passed into the form at all, the value would be "".

I don't know in what context you are using this because it's basically a function definition and you haven't shown any code where it is being invoked.

The first line doesn't make much sense either, since you have gone to the trouble of creating a function to get params from a form and not then using it, this should be:

$CategoryID = get_param('CategoryID');