let count = 0;
let sum = 0;

function javascript() {
    let output = document.querySelector('.response-displayer > .codeblock > pre#javascript');
        output.classList.add('show');
    let nNote = document.querySelector('input[name="cNote"]');

    count++;

    sum = sum + +nNote.value;

    nNote.placeholder = "Entrez la note n°" + ( count + 1 );
    nNote.value = "";

    output.innerHTML = "La moyenne de(s) " + count + " note(s) est actuellement de " + (sum / count);
}

// Already defined inside javascript.js
// let count = 0;
// let sum = 0;

function jquery() {
    $('.response-displayer > .codeblock > pre').removeClass('show')
    let $output = $('.response-displayer > .codeblock > pre#jquery').addClass('show')
    let $nNote = $('input[name="cNote"]');

    count++;

    sum = sum + +$nNote.val();

    $nNote.attr("placeholder", "Entrez la note n°" + ( count + 1 ));
    $nNote.val("");

    $output.html("La moyenne de(s) " + count + " note(s) est actuellement de " + (sum / count) );
}

<?php

if (empty($_SESSION['sum67'])) {
    $_SESSION['sum67'] = 0;
    $_SESSION['count67'] = 0;
}

$_SESSION['count67']++;
$_SESSION['sum67'] = $_SESSION['sum67'] + intval($_POST['cNote']);

unset($_POST['cNote']);

echo "La moyenne de(s) " . $_SESSION['count67'] . " note(s) est actuellement de " . ($_SESSION['sum67'] / $_SESSION['count67']);