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php stop form from posting
<form id = "form" action = "./?page=markandfeedback" method = "post">

    <br>
    Mark for:
<INPUT id="stud" onkeypress="return isNumberKey(event)" type="text" name="stud"  value="Enter Student Number">
<input type="submit" value = 'Continue'>
<?

$studID = $_POST['stud'];
$module2 = $_SESSION['module'];
$ex = $_POST['exer'];
$studerr = array();

$sql = 'SELECT * FROM `student`, `modules` WHERE `studentID` = '.$studID.' AND `moduleCode` = '.$_SESSION['module'];
$result = mysql_query ($sql);
//    echo $_SESSION['module'];

if ($result == NULL) { // nothing found
echo "the student id you entered is not in the database";
}
else {


$_SESSION['student'] = $studID;

Header("Location: http://www.whereever.com/"); // send the browser where you want
exit();
}
?>

<input id="stud" onchange="checkStudentId(this)" />

<script>
function checkStudentId(inputElement) {
 var id = inputElement.value();

  $.ajax({
    url: "test.html",
    context: {id:id}
  }).done(function() {
    // Check the return result
  });
}
</script>

<?
        if(isset($_POST['stud'])){
            $studID = $_POST['stud'];
            $module2 = $_SESSION['module'];
            $ex = $_POST['exer'];
            $studerr = array();
    $host="hostname";//your db host
    $user="user";//your db user
    $pass="pass";//your db pass
    $conn=mysql_connect($host,$user,$pass);

            $sql = 'SELECT * FROM `student`, `modules` WHERE `studentID` = '.$studID.' AND `moduleCode` = '.$_SESSION['module'];
            $result = mysql_query ($sql,$conn);

         if(mysql_num_rows($result)>0){//the id was found in the DB, do whatever here...

        echo $_SESSION['module'];

        $_SESSION['student'] = $studID;

    Header("Location: http://www.whereever.com/");//redirect to wherever
        $error=false;
        }

        else{//id was not found
$error=true;}


        }//end of isset
        ?>


        <? if($error===true){?> <div> The id was not found.... </div> <?}?>
        <form id = "form" action = "<? echo $_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI']; ?>" method = "post">

                <br>
                Mark for:
        <INPUT id="stud" onkeypress="return isNumberKey(event)" type="text" name="stud" value="Enter Student Number">
        <input type="submit" value = 'Continue'>