<script language="javascript">
function checkAll(elementFrm) {
		for (i = 0; i < elementFrm.length; i++)	{
			if(elementFrm.elements[i].type == "checkbox")
				elementFrm.elements[i].checked = true;
		}
	}
			
	function uncheckAll(elementFrm)	{
		for (i = 0; i < elementFrm.length; i++) {
			if(elementFrm.elements[i].type == "checkbox")
				elementFrm.elements[i].checked = false;
		}
	}
	
	function toogle(elementFrm){
		
		var d = 1;
		
		var j = 0;
		
		for(i = 0; i < elementFrm.length; i++){
			if(elementFrm.elements[i].type == "checkbox"){
				j = j + 1;
				if(elementFrm.elements[i].checked == true){
					d = d + 1;
				}
			}
		}
		
		if(d == j){
			uncheckAll(elementFrm);
		} else {
			checkAll(elementFrm);
		}
		
	}
</script>


<form method="post" action="#" name="message_form">


<p>Aanklikken <input type="checkbox" name="check" onClick="toogle(document.message_form)" value="Checken" style="margin: 0px;"/></p>
<p><input type="checkbox" name="messages[]" value="1" style="margin: 0px;"/></p>
<p><input type="checkbox" name="messages[]" value="2" style="margin: 0px;"/></p>
<p><input type="checkbox" name="messages[]" value="3" style="margin: 0px;"/></p>


</form>