PHP variable returns a null value - php

Question

I've created a textbox so when the admin types a name and clicks submit, it will shows a list of retrieved data from the database.
<form method="post" action="">
<?php
$teacherName = $_POST['teacherName'];
if ($_POST['submitted'] == 1) {
if($teacherName != ""){
$getName = mysql_query("SELECT name, user_id FROM members WHERE name = '$teacherName'");
$teacherdetails = mysql_fetch_array($getName);
$teachername = $teacherdetails['name'];
$teacher_id = $teacherdetails['user_id'];
if($teachername != ""){
print $teachername . "<br/>";
} else {
print "Give a valid name <br/>";
}
}
}
if ($teachername == ""){ ?>
Teacher name:<input type="text" size="20" name="teacherName"><input type="hidden" name="submitted" value="1"><br/>
<input type="submit" value="Submit" />
<?php $getModule = mysql_query("......");
while ($row2 = mysql_fetch_array($getModule)) { ?>
<input type="checkbox" name="modules[]" value="<?php print $row2["module_id"]?>"/> <?php print $row2["module_name"] . '<br/>'; } ?>
</div><br/> <?php } ?>
<input type="submit" value="Submit" />
</form>
Below I wrote this code (in the same script):
<?php
$modules = $_POST['modules'];
for($i = 0; $i < count($modules); $i++){
$module=mysql_query("INSERT INTO module (module_id,user_id) VALUES ('$modules[$i]','$teacher_id')");
}
?>
but for some reason when I call the variable "$teacher_id" (which is the value I retrieved before from the database. It works fine in the form) it returns nothing. It's null but I can't understand why.
Am I doing something wrong?

Answer 1

First off, put the PHP outside the form tags, at the top.
Secondly, the data you are receiving could be an array; with more than one result set;
do this just incase it it returned as that;
foreach($teacherdetails AS $teacher) {
//also set the values of the variables here
echo $teacher['name'];
echo $teacher['user_id'];
}
Regarding the last bit, is the $teacher_id successfully printing a result?
Also, where is the modules being input and posted from?