Submit and Generate Dynamic PHP Form

I have a form with text boxes and drop down menus. One of the drop down menus is dependant on the value of another. Eg InventoryUsage is dependent on the value in InventoryID. So far I have done the entire site using php since I do not know javascript. Though i found a javascript function that can get the value entered in InventoryID but I cannot use that value in the php since php is server side. What i need to do is change the second dropdown options depending on that of the first dropdown. Then submit the data as i would with a normal form.

Snippet of Code Below:

<label>TypeOfSurgery</label> <input type="text" name="TypeOfSurgery"          size="35" value="" />
<label>CauseOfSurgery</label> <input type="text" name="CauseOfSurgery" size="35" value="" />
<label>AnaesthesiaUsage</label> <input type="text" name="AnaesthesiaUsage" size="35" value="" />
<label>SurgeryOutcome </label> <input type="text" name="SurgeryOutcome" size="35" value="" />
<label>RecoveryTime</label> <input type="text" name="RecoveryTime" size="35" value="" />
<label>Stages </label> <input type="text" name="Stages" size="35" value="" />
<label>EmployeeName  </label> <p>
<select name="EmployeeName">
<option value=""></option>
<?php
for($i=0;!empty($EmployeeName[$i]);$i++)
echo '<option value="">'.$EmployeeName[$i].'</option>';
?>
</select><p>
<label>Cost</label> <input type="text" name="Cost" size="35" value="" />
<label>InventoryID</label> <p>
<select name="InventoryID">
<option value=""></option>
<?php
for($i=0;!empty($InventoryID[$i]);$i++)
echo '<option value="">'.$InventoryID[$i].'</option>';
?>
</select><p>
<label>InventoryUsage </label> <p>
<select name="InventoryUsage">
<option value=""></option>

<script type="text/javascript">
var model= document.getElementById('InventoryUsage');
</script>
<?php
//if inventory in 
for($i=0;!empty($InventoryUsage[$i]);$i++)
echo '<option value="">'.$InventoryUsage[$i].'</option>';
?>
</select><p>