<script>
function check(ch){
	for(i=0;i<ch.length; i++){
		ch[i].checked = true;
	}
}

</script>
<form>
<input name="yo[]" type="checkbox">
<input name="yo[]" type="checkbox">
<input name="yo[]" type="checkbox">
<input name="yo[]" type="checkbox">
<input type="button" value="Check all" onClick="check(this.form['yo[]'])">
</form>