Insert into an external database using ajax and php : POST 500 (Internal Server Error)

Question

Hello, I try to insert values into my database (in phpmyadmin) hosted by a server (gandi) but this error appear and I don't know how to resolve it My error :

jquery.min.js:4 POST 

http://95.... 500 (Internal Server Error)
send    @   jquery.min.js:4
ajax    @   jquery.min.js:4
(anonymous) @   (index):65
dispatch    @   jquery.min.js:3
q.handle    @   jquery.min.js:3
triggerMouseEvent   @   ionic.bundle.js:2953
tapClick    @   ionic.bundle.js:2942
tapMouseUp  @   ionic.bundle.js:3018

My code :

app.controller('ajoutUtilisateurCtrl', function($scope)
    {
      $('#submit').click(function(){
        var nom = $scope.nom;
        var prenom = $scope.prenom;
        var email = $scope.email;

        var advertisement = $('#nom').val() + $('#prenom').val() + $('#email').val();

        var datastring = {nom: nom, prenom: prenom, email: email};
        if(advertisement=='')
        {
          $('#response').html('Veuillez saisir les données');
        }
        else{
          $.ajax({
            type: "POST",
            url: "http://95.../ajoutUtilisateur.php",
            data: datastring,
            cache: false,
            success: function(result){
              $('#response').html(result);
              var advertisement = $('#nom').val("") + $('#prenom').val("") + $('#email').val("");
            }
          });
          return false;
        }
        })
      });

ajoutUtilisateur.php

<?php
    include_once('config.php');
    $nom = $_POST['nom'];
    $prenom = $_POST['prenom'];
    $email = $_POST['email'];
    $sql = "INSERT INTO utilisateur(nom, prenom, email) VALUES('$nom', '$prenom', '$email')";

    if($connexion->query($sql) === TRUE){
        echo 'Utilisateur ajouté';
    }
    else
    {
        echo 'Erreur ajout utilisateur';
    }
?>

config.php

<?php

    define('DB_HOST', 'localhost');
    define('DB_NAME', 'db');
    define('DB_USERNAME', 'user');
    define('DB_PASSWORD', '');

    $connexion = new mysqli(DB_HOST, DB_USERNAME, DB_PASSWORD, DB_NAME) or die("Could not connect to database");


    if(mysqli_connect_errno()){
        echo('Connexion non établie'.mysqli_connect_error());
        exit();
    }

?>

How can I resolve this problem ?