EXAMPLE USAGE:

<script type="text/javascript">
$(document).ready(function() {
    $('#selectall').click(function(event) {  //on click
        if(this.checked) { // check select status
            $(":checkbox").attr("checked", true);
        }else{
             $(":checkbox").attr("checked", false);
        }
    });
   
});
</script>

<form method="post">
<input type="checkbox" id="selectall" name="selectall">
<input type="checkbox" checked="checked" value="1" name="Test1">
<input type="checkbox" checked="checked" value="2" name="Test2">
<input type="checkbox" checked="checked" value="3" name="Test3">
<input type="checkbox" checked="checked" value="4" name="Test4">
<input type="checkbox" checked="checked" value="5" name="Test5">
 
</form>