//get rid of all kinds of spaces
$IDs = preg_replace('/s+/', '', $inputIDs);
if(!preg_match(''/^\d+$|^(\d+,)+\d+$/'', $IDs)) {
echo "Invalid Input";
}

You could use the validated IDs above in a SQL like the this:

SELECT * FROM t1 WHERE id IN($IDs)

Disclaimer: I am not going to be responsible for any kind of SQL injection resulting from using the snippet above.
updated 20-Mar-2013