Using javascript with php

To all, I have a check box input which I assign an array to the name. I want to pass the array to a javascript and based on a certain criteria check the box. I use the syntax: formname.elements['array[]'][index].checked= true. It works with IE but since my buddy added an html validator to a config file we all use and now the code will not work in Firefox. Does anyone out there perform this task differently? Any suggestions? Sean