If you have an input or textarea and the user writes some stuff in there that contains apostrophes, you won’t be able to process the data with $_GET or $_POST because the apostrophes.
What you can do is to add addslashes like this:
$string = addslashes($_GET[‘var’]);
or
$string = addslashes($_POST[‘var’]);
The var value will go trough, even if it contains apostrophes.