Ezek2k4

function checkAll(checkname, exby) {
  for (i = 0; i < checkname.length; i++)
  checkname[i].checked = exby.checked? true:false
}

//html

<input name="checkall" type="checkbox" id="checkall" value="checkbox" onClick="checkAll(document.poptable.checkGroup,this)"></th>

<input name="checkGroup[]" type="checkbox" id="chksl1" value="11" onclick="msgselect('chksl1')"/></td>

<input name="checkGroup[]" type="checkbox" id="chksl2" value="12" onclick="msgselect('chksl2')"/></td>
<input name="checkGroup[]" type="checkbox" id="chksl3" value="13" onclick="msgselect('chksl3')"/></td>