Populate a textarea from a mysql database where field one is equals drop down value?

I would like to populate a text area with info stored in a MySql database. The catch is.. Well my problem is.. I have a drop down that pulls numbers from the database. This is kind of hard to explain..

Drop down is list of data in "roomnum" field. So if "roomnum" is equal to 555555 the populate textarea with "htmlcode" from same row.

So textarea would equal the htmlcode from the row where number 555555 was the data. I am confusing myself...

<form method="post" action="/backoffice/editor/examples/dump2.php?content=" onsubmit="return validate_form(this)" name="formmy">
Choose Room: <select size="1" name="roomnum" id="roomnum" onchange="">
<?php
 
if ($_POST[roomnum] != "")
{
echo "<option>$_POST[roomnum]</option>";
echo "<option value=\"\"></option>";
}
else
{
echo "<option></option>";
}
 
//require the config file
require ("../../config.php");
 
//make the connection to the database
$connection = @mysql_connect("localhost", "root", "") or die(mysql_error());
$db = @mysql_select_db("pass")or die(mysql_error());
 
//build and issue the query
$sql ="SELECT roomnum FROM rooms WHERE roomnum != 'Template'";
$result = @mysql_query($sql,$connection) or die(mysql_error());
	while ($sql = mysql_fetch_object($result)) 
	{
	    $uname = $sql -> roomnum;
		$beekeeper = $sql -> htmlcode;
	    echo "<option value=\"$uname\">$uname</option>";
	}
?>
</select>
<?php
$formtest = 'roomnum';
$sql ="SELECT htmlcode FROM rooms WHERE roomnum = $formtest";
$result = @mysql_query($sql,$connection) or die(mysql_error());
	while ($sql = mysql_fetch_object($result)) 
	{
	    $uname = $sql -> roomnum;
		$beekeeper = $sql -> htmlcode;
	    
	}
 
?>
 <input type="button" value="Load Page" onClick="">
</p>
<textarea id="codeField" name="codeField" rows="30" cols="80" style="width: 100%" id="codeField"> 
  <?php echo $beekeeper ?>
</textarea>

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