ajax+php的有关问题

ajax+php的问题
有好几个checkbox循环出来，还有个文本框

HTML code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/--><input name="ckID[]" type="checkbox" id="<?='ckID'.$row['MIS_id']?>" value="<?=$row['MIS_id']?>" onclick="showInfor(this.value)"><div id="txtHint"><input name="txtid" type="text" id="txtid"></div>

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JScript code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->function showInfor(str) {     //document.getElementById("txtid").value=str;    //var str = document.geElementById("ckID").value;    //document.write(str);    var xmlhttp;    if (id.length==0)      {      document.getElementById("txtid").value="";      return;      }    if (window.XMLHttpRequest)      {// code for IE7+, Firefox, Chrome, Opera, Safari      xmlhttp=new XMLHttpRequest();      }    else      {// code for IE6, IE5      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");      }    xmlhttp.onreadystatechange=function()      {      if (xmlhttp.readyState==4 && xmlhttp.status==200)        {        //document.getElementById("show").innerHTML=xmlhttp.responseText;        document.getElementById("txtid").value=xmlhttp.responseText;        }      }    xmlhttp.open("GET","showSettle.php?q="+str,true);        xmlhttp.send();}

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PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->$db = mysql_connect("localhost","root","1234");mysql_select_db("it database",$db);$q=$_GET['q']; // connecting to database and stuff...  echo $q;  $temp = mysql_query("SELECT MIS_id FROM materialinstorage WHERE MIS_id = '$q' "); echo $temp;$row = mysql_fetch_array($temp);  $id =$row['MIS_id'];  echo $id;

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HTML code

document.getElementById("txtid").value="";应该是这句，这是什么错啊 //跟你这句没关系的，这句只是设置文本框中的值为空if (id.length==0){  //是这段的处理，主要是id.length这个没有定义，不懂这段做什么，是判断复选框是否被选？ 检查下这段      document.getElementById("txtid").value="";      return;  }<div class="clear">
                 
              
              
        
            </div>

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