计算数组中项目数的问题 - 计数未显示

I'm trying to capture user choices from a multi-select form into PHP so I can match the user choices with what is required. The following is the HTML:

<tr>
<!-- <label for="DutiesDesc">Duties Description:&nbsp;</label><textarea name="DutiesDesc" cols="30" rows="5" /></textarea>-->
<td><label for="DutiesDesc">Job Attributes:</label></td>
<td><select name="DutiesDesc[]" size="<?php $items ?>" multiple="multiple">

<?php while($row = mysql_fetch_array($attribid)) 
{ ?>
<option value="<?php echo $row['AttribName']?>"><?php echo $row['AttribName']?></option>

<?php }?>
   </select></td>
</tr>

The options display correctly, the following part is where I take the user input and store it in an array for later use. Also please note that the displayed multi-select box option are retrieved from a MySQL database.

//Array to store user input from the multiselect box

$SkillsArray = $_POST['DutiesDesc'];
//Count the number of items in the array
$counter = count($SkillsArray);

Just for testing purposes, I count the number of items in the Array and display them as:

echo $counter;

But I get the following error:

Notice: Undefined index: on $counter.

I think this may mean that the $skillsarray doesn't have any values or user selected options didn't save into the variables. Please assist.