Ajax + php problems

Ajax + php has several checkbox loops and a text box HTMLcode & lt; inputname & quot; ckID [] & quot; type & quot; checkbox & quot; ckID. $ row [MIS_id]? & Gt; & quot; value & quot; & lt ;? $ Row [MIS ajax + php problems
There are several checkbox loops and a text box

HTML code

The following is AJAX:

JScript code

  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();}

Then, for the php page

PHP code

  $db = mysql_connect("localhost","root","1234");mysql_select_db("it database",$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;

It seems that the page of php is not executed, and it does not respond.
I execute document. write (str); in ajax to get the data.
What's wrong?

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Get a google browser or firebug!
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Discussion

He said that my id is defined
I changed id. length = 0 in ajax to xmlhttp = null.
The effect is the same, and the webpage error is not displayed in the status bar.

------ Solution --------------------
If (id. length = 0)
This id is not defined
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Step by step debugging. First, alert (xmlhttp. responseText); check whether the php output can be displayed.
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HTML code

Document. getElementById ("txtid "). value = ""; it should be this sentence. what is the error? // it does not matter to you. this sentence only sets the value in the text box to null if (id. length = 0) {// is the processing of this segment, mainly id. length is not defined. I don't know what to do in this section. is it to determine whether the check box is selected? Check the document. getElementById ("txtid"). value = ""; return ;}