<?php
if ($_SERVER['REQUEST_TYPE'] == 'POST') {
    $_POST['optin'] = (isset($_POST['optin'])) : 1 : 0;
    $sql = "INSERT INTO `whatever` (email,optin) VALUES (".$_POST['email'].", ".$_POST['optin'].")";
    mysql_query($sql);
}
?>

<form action="#" method="post">
    <input type="text" name="email">
    <input type="checkbox" name="optin">
    <input type="submit" value="OPSLAAN">
</form>