[insert_php]

 // Initialise variables
$name = "";
$password = "";
$email = "";
$phone = "";
$how = "";
$type = "";

// Sanitise fields
if($_SERVER["REQUEST_METHOD"] == "POST")
{
    $name = test_input($_POST["inputUsername"]);
    $password = test_input($_POST["inputPassword"]);
    $email = test_input($_POST["inputEmail"]);
    $phone = test_input($_POST["inputPhone"]);
    $how = test_input($_POST["inputHow"]);
    $type = test_input($_POST["inputType"]);
}

// Validate fields
registration_validation(
    $name,
    $password,
    $email,
    $phone,
    $how,
    $type
);

// Attempt to submit to database.
complete_registration(
    $name,
    $password,
    $email,
    $phone,
    $how,
    $type
);

// Strip any special characters out of the input.
function test_input($data)
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}

// Do specific checks e.g. length of username etc.
function registration_validation($name, $password, $email, $phone, $how, $type)
{
    global $reg_errors;
    $reg_errors = new WP_Error;

    if (empty($name) || empty($password) || empty($email) || empty($phone) || empty($how) || empty($type))
    {
         $reg_errors->add('field', 'Required form field is missing');
    }

    if (is_wp_error($reg_errors))
    {
        foreach ($reg_errors->get_error_messages() as $error)
        {
            echo '<div>';
            echo '<strong>ERROR</strong>:';
            echo $error . '<br/>';
            echo '</div>';
        }
    }
}

// Submit to the database.
function complete_registration($name, $password, $email, $phone, $how, $type)
{
    global $reg_errors;
    if ( 1 > count( $reg_errors->get_error_messages() ) )
    {
        $userdata = array(
            'user_login'    =>   $name,
            'user_email'    =>   $email,
            'user_pass'     =>   $password,
            'user_phone'    =>   $phone,
            'user_type'     =>   $type,
            'user_how'      =>   $how
        );
        $user = wp_insert_user( $userdata );
        echo 'Registration complete. Goto <a href="' . get_site_url() . '/wp-login.php">login page</a>.';
    }
}
[/insert_php]

<form method="post" action="[insert_php] echo htmlspecialchars($_SERVER["PHP_SELF"]);[/insert_php]">
    <div class="form-group">
      <label for="inputUsername">Username</label>
      <input type="text" class="form-control" id="inputUsername">
    </div>
    <div class="form-group">
        <label for="inputPassword">Password</label>
        <input type="password" class="form-control" id="inputPassword">
    </div>
    <div class="form-group">
      <label for="inputEmail">Email</label>
      <input type="email" class="form-control" id="inputEmail" aria-describedby="emailHelp" placeholder="name@example.com">
    </div>
    <div class="form-group">
        <label for="inputPhone">Contact Phone Number</label>
        <input type="password" class="form-control" id="inputPhone">
    </div>
    <div class="form-group">
        <label for="how">How did you hear about us?</label>
        <select class="form-control" id="how">
            <option>Word of mouth</option>
            <option>Local paper</option>
            <option>Search engine</option>
            <option>Other</option>
        </select>
    </div>
    <div class="form-group">
        <label for="signupType">I am a:</label>
        <select class="form-control" id="signupType">
            <option>Parent / Guardian</option>
            <option>Student</option>
        </select>
    </div>
    <button type="submit" value="Submit" class="btn btn-primary">Submit</button>
  </form>