get url parameter using javascript and send it to php - php

Question

Title is pretty explicit to what I'm trying to do. I want to retreive the variable vavlue in the URL and then send it to add.php.
The reason I don't simply put the PHP code in html is because of a FORM which also send var to add.php
PHP Code :
<?php
$id = $_POST["id"];
?>
Javascript in blabla.html?id=test
<script>
function getResults()
{
var xmlhttp;
if (window.XMLHttpRequest){
xmlhttp=new XMLHttpRequest();
}
else{
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function(){
if (xmlhttp.readyState==4 && xmlhttp.status==200) {
document.getElementById("here").innerHTML=xmlhttp.responseText;
}
}
var id = getParameterByName("id");
var data = "id="+document.getElementById("id").value;
xmlhttp.open("POST","add.php",true);
xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xmlhttp.send(data);
}
</script>
<script>
function getParameterByName(name) {
name = name.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
results = regex.exec(location.search);
return results == null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}
</script>

Answer 1

var data = "id="+document.getElementById("id").value;
^^^^--string containing the letters 'i' and 'd'
should probably be
var data = "id="+document.getElementById(id).value;
^^-- variable named "id"

Answer 2

try this function
function getParameterByName( name ) {
var parts = window.location.search.substr(1).split("&");
var p_arr = {};
for( var i = 0; i < parts.length; i++ ) {
var temp = parts[i].split("=");
if ( decodeURIComponent( temp[0] ) != "" ) {
p_arr[decodeURIComponent( temp[0] )] = decodeURIComponent( temp[1] );
}
}
return p_arr[name];
}
function getResults() {
var xmlhttp;
if ( typeof XMLHttpRequest !== 'undefined' ) {
xmlhttp = new XMLHttpRequest();
}
else {
var versions = [ "MSXML2.XmlHttp.5.0", "MSXML2.XmlHttp.4.0", "MSXML2.XmlHttp.3.0", "MSXML2.XmlHttp.2.0", "Microsoft.XmlHttp" ];
for( var i = 0, len = versions.length; i < len; i++ ) {
try {
xmlhttp = new ActiveXObject( versions[i] );
break;
}
catch(e) {}
}
}
xmlhttp.onreadystatechange = function() {
if ( this.readyState === 4 && this.status == 200 ) {
document.getElementById( "here" ).innerHTML = xmlhttp.responseText;
}
};
var id = getParameterByName( "id" );
var data = "id="+document.getElementById( id ).value;
xmlhttp.open( "POST", "add.php", true );
xmlhttp.setRequestHeader( "Content-Type", "application/x-www-form-urlencoded" );
xmlhttp.send( data );
}