I cannot add data to my table in my mysql database - php

Question

See php code below:
I have built the html form and the dropdown menu in the html form using <<<_END _END tags in php. also I added the following php code at the top of the htmlform that I had believed would allow me to enter a student name, student ID and select a course from the dropdown menu in the form. once those 3 values were entered in the form, the info should be added to the table enrolment in my mysql database. but i have had no luck figuring this out...
//connect3.php--login information to my mysql database//
<?php
define ('HOST', 'localhost');
define ('USER', 'root');
define ('PASS', '******');
?>
// html form and connection code//
<?php
include 'connect3.php';
$link = mysql_connect (HOST, USER, PASS) or die(mysql_error());
mysql_select_db ('milleruniversity', $link);
// Added mysql_real_escape() to protect against SQL-Injection
$code = mysql_real_escape( $_POST['code'] );
$uid = mysql_real_escape( $_POST['uid'] );
$studentname = mysql_real_escape( $_POST['studentname'] );
// Insert a row of information into the table "enrolment"
$query = "INSERT INTO enrolment (code, uid, studentname) VALUES('$code', '$uid', '$studentname')";
if(mysql_query($query)){
echo "inserted";}
else{
echo "fail";}
echo <<<_END
<table border='1'cellpadding="10">
<tr>
<td>
<h4>Miller University Registration Form</h4>
<p>Please Register as a new Student or current student for the following courses below.</p>
</td>
</tr>
<form action="draft5.php" method="post"><pre>
<tr>
<td>
Student Name <input type="text" name="studentname" maxlength="30"/>
</td>
</tr>
<tr>
<td>
Student ID <input type="text" name="uid" maxlength="11"/>
</tr>
</td>
<tr>
<td>
Select a course <select name="code" size="1">
<option value="DC-00040">Digital Communications</option>
<option value="VC-00030">Visual Culture</option>
<option value="WP-00080">World Politics</option>
</select>
</tr>
</td>
<tr>
<td>
<input type="submit" value="Submit to Register" />
</tr>
</td>
</pre></form>
</table>
_END;
mysql_close($link);
?>

Answer 1

It seems to me that you use this draft5.php page to display the form and to insert a row in your database. In that case the problem may come from the missing isset() around the $_POST data. (the first time you load the page the POST values are not set).
You could use this code :
if( (isset($_POST['code']) AND (isset($_POST['uid']) AND (isset($_POST['studentname']){
//process the data base treatment and display an acknoledgment of the insert
// Check if these new code, uid and studentname respect the primary key constraint
}
else{
// Display your form
}
You could also consider using backticks ` around the names of your tables and colomns.
If you want to prevent the same student to register twice in the same course you need to add primary keys in your tables. But this is not enough, indeed if you perform the insert request with a contraint violation on the primary key MySql will return an error. What you can do is check if the key exists prior to the insert request, if yes notify the user, if no perform the insert request.