<SCRIPT LANGUAGE="JavaScript">
 //判断同类型
 function checkboxnum(){
  var form = document.forms['formbj1'];
  var i,j=0;
  for (i=0; i<form.length; i++){
  var e=form[i];
  if (e.checked && e.type=='checkbox' && e.name=='bjid[]') {
   j++;
   if(j==3){
  alert("最多只能选择2项！");
  return false;
  break;
   }
  }
  }
 }
 </script>


<form action="zhdk_bj.php" method="post" name="formbj1" id="formbj1">

<input name="bjid[]" id="bjid[]" type="checkbox" onclick="return checkboxnum()" value="1" />

<input name="bjid[]" id="bjid[]" type="checkbox" onclick="return checkboxnum()" value="2" />

<input name="bjid[]" id="bjid[]" type="checkbox" onclick="return checkboxnum()" value="3" />


</form>