<?php
$errors = [];

if(!array_key_exists('user_name', $_POST) || $_POST['user_name'] == ''){
    $errors['user_name'] = "il manque votre nom";
}
if(!array_key_exists('user_email', $_POST) || $_POST['user_email'] == '' || !filter_var($_POST['user_email'], FILTER_VALIDATE_EMAIL)){
    $errors['user_email'] = "il manque votre email";
}
if(!array_key_exists('user_message', $_POST) || $_POST['user_message'] == ''){
    $errors['user_message'] = "il manque votre message";
}
if(!array_key_exists('user_tel', $_POST) || $_POST['user_tel'] == ''){
    $errors['user_tel'] = "il manque votre tel";
}
if(!array_key_exists('user_select', $_POST) || $_POST['user_select'] == ''){
    $errors['user_select'] = "il manque votre selection";
}
if(!empty($errors)){
    session_start();
    $_SESSION['errors'] = $errors;
    $_SESSION['inputs'] = $_POST;
    //echo "<pre>"; var_dump($_POST);echo "<pre>";die();
    header('location: form.php');
}

var_dump($errors);
die();