When using AJAX / JavaScript in your form, you'd better not use names/id's with [] at the end. It will make the form element inaccesibile for the JavaScript function document.getElementById() and you won't be able to address the element with document.mywebform.name[]. Solution: save the selected options as a string in a comma seperated list, and make this string the value of a hidden input element. In your php script, explode() this string