select all check box in php

script type="text/javascript">
checked=false;
function checkedAll (frm1) {
var aa= document.getElementById('frm1');
if (checked == false)
{
checked = true
}
else
{
checked = false
}
for (var i =0; i < aa.elements.length; i++)
{
aa.elements[i].checked = checked;
}
}
/script>

form id ="frm1"
input type="checkbox" name="chk1"
input type="checkbox" name="chk2"
/form>