client.js

function ajaxFunction(){
	var ajaxRequest;  // The variable that makes Ajax possible!
	
	try{
		// Opera 8.0+, Firefox, Safari
		ajaxRequest = new XMLHttpRequest();
	} catch (e){
		// Internet Explorer Browsers
		try{
			ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
		} catch (e) {
			try{
				ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
			} catch (e){
				// Something went wrong
				alert("Your browser broke!");
				return false;
			}
		}
	}
	// Create a function that will receive data sent from the server
	ajaxRequest.onreadystatechange = function(){
		if(ajaxRequest.readyState == 4){
			//document.chronoform_devis.getElementById('partmail').innerHTML = ajaxRequest.responseText;
			alert('ok');
		}
	}
	//var codepart = document.getElementById('codepart').value;
	
	ajaxRequest.open("GET", "http://www.courtier-en-pret-immobilier.com/partenaires.php", true);
	ajaxRequest.send(null); 
}

server.php

<?php
$dbhost = "xxxx";
$dbuser = "xxxx";
$dbpass = "xxxx";
$dbname = "xxxx";
	//Connect to MySQL Server
mysql_connect($dbhost, $dbuser, $dbpass);
	//Select Database
mysql_select_db($dbname) or die(mysql_error());

	// Retrieve data from Query String
	
$codepart = $_GET['codepart'];
	
// Escape User Input to help prevent SQL Injection

$codepart = mysql_real_escape_string($codepart);

//build query
$query = "SELECT partMail FROM partenaires WHERE codepart = '$codepart'";

if ($result=mysql_query($query)) {
			$mailpart= mysql_num_rows($result);
		} else {
			$message = "<p><font color=\"red\"><b>Le code partenaires n'existe pas.<br /> ".$query."</b></font></p>";
		}
		
	echo $mailpart;	
		
		mysql_free_result($result);
		mysql_close($db);



?>