look up http://www.php.net/addslashes
<?
$userdata = $_POST['username'];
$userdata = addslashes($userdata);
// insert into database here
?>
$userdata = addslashes($userdata); // changes " to \", ' to \' etc.
// so John O'Connor would become John O\'Connor
note: when you try to read this value now you wont want to see the \, so you have to look up addslashes opposite, http://www.php.net/stripslashes
<?
// read from database
$userdata = $arr['username'];
$userdata = stripslashes($userdata);
echo $userdata; // returns John O'Connor
?>
hth