Using Option.length With Php Array In Form

Ok the setup is basically this. The user selects a option in a drop down field, whatever they select through javscript additional records are populated in a second select field. This second select field allows for multiple selections, however I am submitting the form to a php script and I want to capture the multiple selections in the second select field. To do that for php I need to turn the field into an array so I have to add the brackets, [], to the name of the select field.

In php you can name the field like so:

Code:
<select name="fld[]" size="1">

The problem is in the js function when I give it a field name with [] the script doesnt work. Here is the js function:

Code:
function channelform(select_value) {
IntPath = document.channel_form.cid[]
TheOptions = IntPath.options.length

[Code]..

You can see the [] for the cid field name. Whenever I add that to the field name the js script is broken. How do I get it to work so that I can pass an array to php when the form submits?