<button type="button" name="sign_up" id="signup-btn" class="btn btn-primary btn-block">Signup</button>

$(document).ready(function(){
    $("#signup-btn").click(function(){
        var f_name      = $("#f_name").val();
        var l_name      = $("#l_name").val();
        var username    = $("#username").val();
        var email       = $("#u_email").val();
        var password    = $("#password").val();
        var country     = $("#sel1").val();
        var gender      = $("#gender1").val();
        var birthday    = $("#inputBirthday").val();
        var acc_type    = $("#optionsRadios1").val();

        var dataString =
        'f_name='+ f_name +
        '&l_name='+ l_name +
        '&username='+ username +
        '&u_email='+ email +
        '&u_pass='+ password +
        '&u_country='+ country +
        '&u_gender='+ gender +
        '&u_birthday='+ birthday +
        '&account_type='+ acc_type;

        if(username==''||email==''||password=='')
        {
            alert("Please Fill All Fields");
        }
        else
        {
        // AJAX Code To Submit Form.
        $.ajax({
            type: "POST",
            url: "user_insert.php",
            data: dataString,
            cache: false,
            success: function(data){
                alert(data);
            }
        });
    }
    return false;
});
});

<?php
include ("includes/connection.php");
if (isset($_POST['sign_up'])) {

    $f_name = mysqli_real_escape_string($con, $_POST['f_name']);
    $l_name = mysqli_real_escape_string($con, $_POST['l_name']);
    $username = mysqli_real_escape_string($con, $_POST['username']);
    $email = mysqli_real_escape_string($con, $_POST['u_email']);
    $pass = mysqli_real_escape_string($con, $_POST['u_pass']);
    $country = mysqli_real_escape_string($con, $_POST['u_country']);
    $gender = mysqli_real_escape_string($con, $_POST['u_gender']);
    $birthday = mysqli_real_escape_string($con, $_POST['u_birthday']);
    $acc_type = mysqli_real_escape_string($con,$_POST['account_type']);
}
}
?>