PHP - Editar un formulario de contacto para que se pueda seleccionar el destinatario

<form id="pp_booking_form" method="post" action="/wp-admin/admin-ajax.php">

    <input type="hidden" id="action" name="action" value="pp_booking_mailer"/>

    <input type="hidden" id="tour_title" name="tour_title" value="<?php echo get_the_title(); ?>"/>

    <input type="hidden" id="tour_url" name="tour_url" value="<?php echo get_permalink($current_page_id); ?>"/>

    <div class="one_half">

        <label for="first_name"><?php echo _e( 'First Name', THEMEDOMAIN ); ?></label>

        <input id="first_name" name="first_name" type="text" class="required_field"/>

    </div>

    <div class="one_half last">

        <label for="last_name"><?php echo _e( 'Last Name', THEMEDOMAIN ); ?></label>

        <input id="last_name" name="last_name" type="text" class="required_field"/>

    </div>

    <br class="clear"/><br/>

    <div class="one_half">

        <label for="email"><?php echo _e( 'Email', THEMEDOMAIN ); ?></label>

        <input id="email" name="email" type="text" class="required_field"/>

    </div>

    <div class="one_half last">

        <label for="phone"><?php echo _e( 'Phone', THEMEDOMAIN ); ?></label>

        <input id="phone" name="phone" type="text"/>

    </div>

    <br class="clear"/><br/>

    <div class="one">

        <label for="message"><?php echo _e( 'Additional Message', THEMEDOMAIN ); ?></label>

        <textarea id="message" name="message" rows="7" cols="10"></textarea>

    </div>

    <br class="clear"/>

    <div class="one">

        <p>

            <input id="booking_submit_btn" type="submit" value="<?php echo _e( 'Book By Email', THEMEDOMAIN ); ?>"/>

        </p>

    </div>

</form>

/** 

*    Setup booking form mailing function 

**/

add_action('wp_ajax_pp_booking_mailer', 'pp_booking_mailer');

add_action('wp_ajax_nopriv_pp_booking_mailer', 'pp_booking_mailer');

function pp_booking_mailer() {

    check_ajax_referer( 'tgajax-post-contact-nonce', 'tg_security' );

    //Get your email address 

    $contact_email = get_option('pp_booking_email');

    $pp_contact_thankyou = '<br/>'.__( 'Thank you for booking! We will get back to you as soon as possible', THEMEDOMAIN );

    $pp_contact_thankyou.= '<br/><br/><a href="javascript:;" id="booking_close_form" class="button">'.__( 'Close', THEMEDOMAIN ).'</a>';

    //Error message when message can't send 

    define('ERROR_MESSAGE', 'Oops! something went wrong, please try to book later.');

    if (isset($_POST['first_name'])) {

        /* 

        | 

        | Begin sending mail 

        | 

        */

        $first_name = $_POST['first_name'];

        $last_name = $_POST['last_name'];

        $email = $_POST['email'];

        $phone = $_POST['phone'];

        $form_message = $_POST['message'];

        $tour_title = $_POST['tour_title'];

        $tour_url = $_POST['tour_url'];

        $from_name = $first_name.' '.$last_name;

        //Get contact subject 

        if(!isset($_POST['subject']))

        {

            $contact_subject = __( 'Booking form', THEMEDOMAIN );

        }

        else

        {

            $contact_subject = $_POST['subject'];

        }

        $headers = "";

           $headers.= 'From: '.$from_name.' <'.$email.'>'.PHP_EOL;

           $headers.= 'Reply-To: '.$from_name.' <'.$email.'>'.PHP_EOL;

           $headers.= 'Return-Path: '.$from_name.' <'.$email.'>'.PHP_EOL;

        $message = __( 'Booking for', THEMEDOMAIN ).' '.$tour_title.' ('.$tour_url.')'.PHP_EOL;

        $message.= 'Name: '.$from_name.PHP_EOL;

        $message.= 'Email: '.$email.PHP_EOL.PHP_EOL;

        $message.= 'Phone: '.$phone.PHP_EOL.PHP_EOL;

        $message.= 'Message: '.PHP_EOL.$form_message.PHP_EOL.PHP_EOL;

        if(!empty($first_name) && !empty($last_name) && !empty($email))

        {

            wp_mail($contact_email, $contact_subject, $message, $headers);

            echo '<p>'.$pp_contact_thankyou.'</p>';

            die;

        }

        else

        {

            echo '<p>'.ERROR_MESSAGE.'</p>';

            die;

        }

    }

    else

    {

        echo '<p>'.ERROR_MESSAGE.'</p>';

    }

    die();

}