Xml Document Won't Update Using PhP Form

I write a small program that is supposed to update an xml file (KPI.xml) with an html form. I compared my code with everything I found o internet and haven't been able to find why my xml doc doesn't change when I complete the form. PHP doesn't return me any error.

Here is my code:

 <form method='post'action=''>
    <label>Mois</label>
    <input type='text' name='mois'/>
    <br>

    <label>Jour</label>
    <input type='text' name='jour'/>
    <br>
    <label>RNC</label>
    <input type='text' name='rnc'/>
    <br>
    <label>Production</label>
    <input type='text' name='production'/>
    <br>
    <label>DROP</label>
    <input type='text' name='DROP'/>
    <br>


    <input type='submit' value='submit'/>

</form>

    <?php
if (isset($_POST['submit'])) {
    $xmldoc = new DOMDocument('1.0');
    $xmldoc ->load("KPI.xml");

    $mois = $_POST['mois'];
    $Jour = $_POST['jour'];
    $rnc = $_POST['rnc'];
    $Prod = $_POST['production'];
    $DROP = $_POST['DROP'];



        // find the headercontent tag
        //$root = $xmldoc->getElementsByTagName('Usine')->item(0);

        // create the <product> tag
        $KPI = $xmldoc->createElement('KPI');
        $KPI ->setAttribute('mois', $mois);



        // create other elements and add it to the <KPI> tag.
        $jourElement = $xmldoc->createElement('Jour');
        $jourText = $xmldoc->createTextNode($Jour);
        $KPI->appendChild($jourElement);
        $jourElement->appendChild($jourText);


        $rncElement = $xmldoc->createElement('RNC');
        $rncText = $xmldoc->createTextNode($rnc);
        $KPI->appendChild($rncElement);
        $rncElement->appendChild($rncText);

        $DROPElement = $xmldoc->createElement('DROP');
         $DROPText = $xmldoc->createTextNode($DROP);
        $KPI->appendChild($DROPElement);
        $DROPElement->appendChild($DROPText);

        $ProdElement = $xmldoc->createElement('Prod');
        $ProdText = $xmldoc->createTextNode($Prod);
        $KPI->appendChild($ProdElement);       
        $ProdElement->appendChild($ProdText);

         // add KPI to the DOM
        $xmldoc->documentElement->appendChild($KPI);

        $xmldoc->save("KPI.xml");

}
?>