show javascript alert box when a field is empty, if not show form elements in a table in another php file

its my first php exercice and i am asked to make a form in a php file and show result in another file i chose the post method
then i have been asked to show a javascript alert if a field is empty
the fist part was fine, but im struggling to mix php js in the submit button for the second part. i tried changing order of code i put the php and script below the form but it still didnt work
file 1

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>ex4</title>
</head>
<body>
    <?php 
    function check_input(){
        if ($_SERVER["REQUEST_METHOD"] == "POST") {
            if (empty($_POST['nom']) || empty($_POST['prenom']) || empty($_POST['adresse']) || empty($_POST['code_postal'])){
                echo "window.alert("un ou plusieurs champs ne sont pas remplis !");";
            }
        }
    }
    ?>
    <script>
        function myFunction() {
        <?php check_input(); ?>
        }
    </script>

    <form method="post" action="ex4sep.php">
        <fieldset style ="display: inline-block;">
        <legend>Informations Etudiants</legend>
        <table>
            <tr>
                <td style="width:50%;">nom</td>
                <td><input type="text" name="nom"></td>
            </tr>
            <tr>
                <td style="width:50%;">prenom</td>
                <td><input type="text" name="prenom"></td>
            </tr>
            <tr>
                <td style="width:50%;">adresse</td>
                <td><input type="text" name="adresse"></td>
            </tr>
            <tr>
                <td style="width:50%;">code postal</td>
                <td><input type="text" name="code_postal"></td>
            </tr>
            <tr>
            <td colspan="2"><input type="submit" onclick="myFunction"></td>
            <td></td>
            </tr>
        </table>
    </fieldset>
    </form>
</body>
</html>

file 2 of results

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>ex4sep</title>
</head>
<body>
    <table border>
            <tr>
                <td>nom</td>
                <td><?php echo $_POST['nom']; ?></td>
            </tr>
            <tr>
                <td>prenom</td>
                <td><?php echo $_POST['prenom']; ?></td>
            </tr>
            <tr>
                <td>adresse</td>
                <td><?php echo $_POST['adresse']; ?></td>
            </tr>
            <tr>
                <td>code postal</td>
                <td><?php echo $_POST['code_postal']; ?></td>
            </tr>
        </table>
</body>
</html>