Passage de variable javascript à php

<form name="form1" method="post" action="script.php" onSubmit="return ecrire_valeur('une valeur');">
....autres cahmps
<inupt type="hidden" name="champ1" id="champ1" value="">
<input type="submit" name="envoyer" value="Envoyer">
<input
</form>

function ecrire_valeur(e){
obj=document.getElementById('champ1');
obj.value=e;
return true; //ça c'est pour que le formulaire parte


}

<?php
session_start();
$_SESSION['variable_javascript'] = $_POST['value'];
?>

function valeur(e){
    var v = e.innerHTML;
    return v;
} 

function getXMLHttpRequest() {
	var xhr = null;
	
	if (window.XMLHttpRequest || window.ActiveXObject) {
		if (window.ActiveXObject) {
			try {
				xhr = new ActiveXObject("Msxml2.XMLHTTP");
			} catch(e) {
				xhr = new ActiveXObject("Microsoft.XMLHTTP");
			}
		} else {
			xhr = new XMLHttpRequest(); 
		}
	} else {
		alert("Votre navigateur ne supporte pas l'objet XMLHTTPRequest...");
		return null;
	}
	
	return xhr;
}

function onload_ajax() {
    var ajax = getXMLHttpRequest();
    ajax.open("POST", "ajax-variable-js.php", true);
    ajax.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    ajax.send("value=" + valeur(e) );
}

<head>
    <!-- Avant -->
    <script type="text/javascript" src="ajax-variable-js.js"></script>
    <!-- Après -->
</head>
...

<body onload="onload_ajax()">

<?php session_start(); ?>