<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>