if(isset($_POST['nom']) && empty($_POST['nom'])) {

  echo "<script>


      window.onload = function() {

        alert('Vous devez entrer un nom !');

      }




  </script>
  ";
}



  if(isset($_POST['mail']) && empty($_POST['mail'])) {

    echo "
    <script>


    window.onload = function(){

      alert('Vous devez entrer un mail !')
    }

</script>

    ";
  }


  if(isset($_POST['message']) && empty($_POST['message'])) {

    echo "

    <script>

    window.onload = function(){

      alert('Vous devez entrer un message !')
    }
</script>

    ";
  }