how to put values from database into combobox

I am using php and i have made two combo boxes.Actually i want that whenever user select any value from combo box1 named subject the values from database are automatically displayed in the combobox2 named supervisor.But whenever i run the code it gives error as "undefined index subject" and values are not retrieved in the second combobox.here is the code

Code:

<form method="post" action="" name="form1">

<table width="60%" border="0" cellspacing="0" cellpadding="0">
  <tr>
    <td width="150"></td>
    <td  width="150">
	  <p>
	    <select name="subject" >
	        <option value="">Select Subject</option>
	        <option value="1">Networking</option>
	        <option value="2">Databases</option>
	        <option value="3">Web development</option>
	        <option value="4">digital signal processing</option>
	      </select>
		  
		  
	  </p>
	</td>
  </tr>

  </table>
  
  <?php
include("conpage.php");
$s1= intval($_POST['subject']); //here it gives error as subject to be        undefined
mysql_select_db('thesis');
$query="SELECT id,supervisorname FROM supervisors WHERE subjectid='$s1'";   //supervisors table from where values in the second combo box have to be displayed
$result=mysql_query($query);

?>


<select name="state">
<option>Select Supervisor</option>
<? while($row=mysql_fetch_array($result)) { ?>
<option value=<?=$row['id']?>><?=$row['supervisorname']?></option>
<? } ?>
</select>
</form>

i donot know how to solve this?

 

Please add

Code:

print_r($_POST);

to your code to see how the $_POST array looks:

Code:

<form method="post" action="" name="form1">

<table width="60%" border="0" cellspacing="0" cellpadding="0">
  <tr>
    <td width="150"></td>
    <td  width="150">
	  <p>
	    <select name="subject" >
	        <option value="">Select Subject</option>
	        <option value="1">Networking</option>
	        <option value="2">Databases</option>
	        <option value="3">Web development</option>
	        <option value="4">digital signal processing</option>
	      </select>
		  
		  
	  </p>
	</td>
  </tr>

  </table>
  
  <?php
include("conpage.php");
[COLOR="Red"]print_r($_POST);[/COLOR]
$s1= intval($_POST['subject']); //here it gives error as subject to be        undefined
mysql_select_db('thesis');
$query="SELECT id,supervisorname FROM supervisors WHERE subjectid='$s1'";   //supervisors table from where values in the second combo box have to be displayed
$result=mysql_query($query);

?>