A scopo puramente didattico 
Codice PHP:
function checkMe (pItem, pForm) {
var _form= document.getElementById(pForm);
if (pItem.checked) {
for (var i=0;i< _form.elements.length;i++) {
if (document.getElementById(pForm).elements[i].name.substring(0,4) == "chk_") {
document.getElementById(pForm).elements[i].checked = true;
}
}
} else {
// Deseleziona tutto
for (var i=0;i< _form.elements.length;i++) {
if (document.getElementById(pForm).elements[i].name.substring(0,4) == "chk_") {
document.getElementById(pForm).elements[i].checked = false;
}
}
}
}