Why is this form field not showing up in results email? (php/java question)?

[michaelnaeseth]

I have a form with a hidden field that becomes visible when user clicks a specific checkbox. The problem is that the user input from this hidden form field is never included in the form results email. I am using Javascript to perform the show/hide tasks...... relevant code below

<script language="JavaScript">
function showhidefield()
{
if (document.frm.systemtype_f.checked)
{
document.getElementById("hideablearea").style.visibility = "visible";
}
else
{
document.getElementById("hideablearea").style.visibility = "hidden";
}
}
</script>


</head>

<form action="action_post.php" method="POST" name="frm" target="_self">

<input type="checkbox" checked="checked" name="systemtype_a" value="ac">A/C System
<input type="checkbox" name="systemtype_h" value="heatpump">Heat Pump
<input type="checkbox" name="systemtype_f" onclick="showhidefield()" value="furnace">Furnace

<div id='hideablearea' style='visibility:hidden;'>Furnace type: <select name="fur_type">
<option value="electric">Electric</option><option value="naturalgas">Natural Gas</option><option value"propane">Propane Gas</option></select></div>
<br /><b>Preferred project completion date:</b> Â* Â*
<select name="completion_date"><option value="flexible">Flexible</option><option value="1_week">Within 1 week</option><option value="2_week">Within 1-2 weeks</option><option value="3_week">More than 2 weeks</option></select><br /><br /><br /><br /><br />
<b>My Zip Code: </b> Â* Â* <input type="text" style="font-size:14px" size="5" name="zip" maxlength="5" /> Â*
</div>
<center><input type="submit" value="Give me my 3 estimates!" /></center>
</form>

All other user entered variables show up correctly on the results email. I am only having problems with the $fur_type select box. Please help.