yes, i know there is already this kind of question, but i still don't get what i'm doing wrong...
Basically i'm doing a simple Ajax post in jquery to a PHP page, but the xhr status returns me HTTP 500
here the code:
    function Register(mailval) {
    var dataString = 'email1=' + mailval;
AJAX
    // AJAX code to submit form.
    $.ajax({
    type: "POST",
    url: "register.php",
    data: dataString,
    cache: false,
    success: function(data) {
        alert(data);
        $(".modal-body").html('');
        $(".modal-body").html(data);
    },
      error: function (xhr, ajaxOptions, thrownError) {
        console.log(xhr.status);
        console.log(thrownError);
      }     
    });
}
PHP
<?php
$email2 = $_POST['email1'];
$connection = mysqli_connect("127.0.0.1", "user", "psw"); 
$db = mysql_select_db("wnarshix_contatti_form", $connection);
if (isset($_POST['email1'])) {
    $query = mysql_query("insert into user_registered(mail) values ('$email2')");
    echo "Grazie per esserti registrato";
}
mysqli_close($connection); 
?>
i'm not using mysqli properly?
 
    