communication with database through ajax - php

Question

i have following set of table rows which i have fetched from the database
echo "<td value='".$id."' style='border:double' bgcolor='" . getcolour($catc) . "' onclick = 'addValue(this.value);'>$id</td>";
this is actually a set of boxes and $id = $row['id'] is the number attached to each box. I want the mechanism like when a user clicks on the box the number attached to it should be echoed on a particular place on the webpage. MySql query and everything else works fine i have a problem with ajax code understanding i think so.
<script>
function valueAdd(str)
{
if (str=="")
{
document.getElementById("seats").innerHTML="";
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("seats").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","ceckbus.php?q="+str,true);
xmlhttp.send();
}
</script>
waiting for ur help everyone!!

Answer 1

First I suggest that you use a javascript library in order to use Ajax, and avoid trying to fix the compatibility issues at your own. For example you could use jquery.com, it is pretty standard nowadays.
For the question part, please have a look at your echo code, there is an error at 'addValue(this.value);'
As I can understand the question, you want on the click event to have the $id. A simple fix it would be by using the innerHTML property, instead on .value:
echo "<td value='".$id."' style='border:double' bgcolor='" . getcolour($catc) . "' onclick = 'addValue(this.innerHTML);'>$id</td>";
or better on the echo you could pass the value directly to the addValue function, eg:
echo "<td value='".$id."' style='border:double' bgcolor='" . getcolour($catc) . "' onclick = 'addValue(\"" . $id . "\");'>$id</td>";