codice:
function select_all(obj){
  var ff = obj.form;
  var cc = ff.elements['myCheckbox[]'];
  for (var i=0;i<ff.elements.length;i++) {
    el = ff.elements[i];
    if (el.type=="checkbox" && el.name=="myCheckbox[]" && (!el.disabled)) 
      el.checked = obj.checked;
  }
}
da usare così
codice:
<form>
  <input type="checkbox" name="checkAll" value="on" onclick="select_all(this)">

  <input type="checkbox" name="myCheckbox[]" value="1">
  <input type="checkbox" name="myCheckbox[]" value="2">
  <input type="checkbox" name="myCheckbox[]" value="3">
  <input type="checkbox" name="myCheckbox[]" value="4">
  <input type="submit" value="submit">
</form>