Data is not inserted correctly in MySQL database

I try to insert data from a form into a database. When I fill in the form the code is executed correctly and does not give an error. I have a variable ID that auto increments and every time I execute the code a new record appears but in the other fields they all appear blank. Thanks for your help.

It should come out like this: It goes like this:

This is my code:

<?php
$link = mysqli_connect("localhost", "us", "ps", "db");

if($link === false){
die("ERROR: No se puede conectar. " . mysqli_connect_error());
}

$first_name = mysqli_real_escape_string($link, $_REQUEST['first_name']);
$last_name = mysqli_real_escape_string($link, $_REQUEST['last_name']);
$last_name2 = mysqli_real_escape_string($link, $_REQUEST['last_name2']);
$campus = mysqli_real_escape_string($link, $_REQUEST['campus']);
$phone = mysqli_real_escape_string($link, $_REQUEST['phone']);
$email = mysqli_real_escape_string($link, $_REQUEST['email']);
$age = mysqli_real_escape_string($link, $_REQUEST['age']);
$pay = mysqli_real_escape_string($link, $_REQUEST['pay']);

$sql = "INSERT INTO registro (first_name, last_name, last_name2, campus, phone, email, age, pay) VALUES ('".$first_name."', '".$last_name."', '".$last_name2."', '".$campus."', '".$phone."', '".$email."', '".$age."', '".$pay."')";

if(mysqli_query($link, $sql)){
echo "Su registro se ha realizado correctamente.";
} else{
echo "ERROR: no se pudo añadir $sql. " . mysqli_error($link);
}
// close connection
mysqli_close($link);
?>

This is the code of the form:

<form class="form-horizontal" action="enviar.php" id="registro" name="registro" method="POST">
            <div class="form-group">
                <label class="control-label col-sm-2" for="name">Nombre:</label>
                <div class="col-sm-10">
                    <input type="text" class="form-control" id="first_name" placeholder="Ingresa tu Nombre" required>
                </div>
            </div>
            <div class="form-group">
                <label class="control-label col-sm-2" for="name">Apellido Paterno:</label>
                <div class="col-sm-10">
                    <input type="text" class="form-control" id="last_name" placeholder="Ingresa tu Apellido Paterno"required>
                </div>
            </div>
            <div class="form-group">
                <label class="control-label col-sm-2" for="name">Apellido Materno:</label>
                <div class="col-sm-10">
                    <input type="text" class="form-control" id="last_name2" placeholder="Ingresa tu Apellido Materno" required>
                </div>
            </div>
            <div class="form-group">
                            <div class="col-sm-offset-2 col-sm-10">
                                <label for="select">Selecciona tu Campus:</label>
                                <select required class="form-control" id="campus">
                                    <option>Ensenada</option>
                                    <option>Tijuana</option>
                                    <option>Mexicali</option>
                                    <option>Ciudad Obregon</option>
                                    <option>Hermosillo</option>
                                    <option>Nogales</option>
                                    <option>Ciuada Juarez</option>
                                    <option>Chihuahua</option>
                                    <option>Saltillo</option>
                                    <option>Torreon</option>
                                    <option>Guasave</option>
                                    <option>Culiacan</option>
                                    <option>Los Mochis</option>
                                    <option>Mazatlan</option>
                                    <option>Durango</option>
                                    <option>Santiago Papasquiaro</option>
                                    <option>Laguna</option>
                                    <option>Nuevo Leon</option>
                                    <option>Zacatecas</option>
                                    <option>Fresnillo</option>
                                    <option>Michoacan</option>
                                    <option>Aguascalientes</option>
                                    <option>Queretaro</option>
                                    <option>Guanajuato</option>
                                    <option>Mexico</option>
                                    <option>Hidalgo</option>
                                </select>
                            </div>
                        </div>
                        <div class="form-group">
                            <label class="control-label col-sm-2" for="phone">Telefono:</label>
                            <div class="col-sm-10">
                                <input type="text" class="form-control" id="phone" placeholder="Ingresa tu Telefono" required
                            </div>
                        </div>
                        <div class="form-group">
                            <label class="control-label col-sm-2" for="email">Email:</label>
                            <div class="col-sm-10">
                                <input type="email" class="form-control" id="email" placeholder="Ingresa tu email" required>
                            </div>
                        </div>
                        <div class="form-group">
                            <label class="control-label col-sm-2" for="age">Edad:</label>
                            <div class="col-sm-10">
                                <input type="number" min="18" max="60" class="form-control" id="age" placeholder="Ingresa tu Edad" required>
                            </div>
                        </div>
                        <div class="form-group">
                            <label class="control-label col-sm-2" for="pay">Clave de Pago:</label>
                            <div class="col-sm-10">
                                <input type="text" class="form-control" id="pay" placeholder="Ingresa la Clave de Pago" required>
                            </div>
                        </div>
                        <div class="form-group">
                            <div class="col-sm-offset-2 col-sm-10">
                                <button type="submit" class="btn btn-info">Enviar</button>
                                <button type="reset" class="btn btn-danger">Borrar</button>
                            </div>
                        </div>
                    </form>