ATmega32U4 code:

#include <Mailbox.h>
void setup() {
  pinMode(13, OUTPUT);
  digitalWrite(13, LOW);
  // Initialize Bridge and Mailbox
  Bridge.begin();
  Mailbox.begin();
  digitalWrite(13, HIGH);
  // Initialize Serial
  Serial.begin(9600);
  // Wait until a Serial Monitor is connected.
  while (!Serial);
  Serial.println("Mailbox Read Message\n");
  Serial.println("The Mailbox is checked every 10 seconds. The incoming messages will be shown below.\n");
}
void loop() {
  String message;
  // if there is a message in the Mailbox
  if (Mailbox.messageAvailable())
  {
    // read all the messages present in the queue
    while (Mailbox.messageAvailable())
    {
      Mailbox.readMessage(message);
      Serial.println(message);
    }
    Serial.println("Waiting 10 seconds before checking the Mailbox again");
  }
  // wait 10 seconds
  delay(10000);
}

AR9331 code:

opkg update
opkg install php5-cli php5-mod-sockets php5-mod-json
nano mailbox.php

<?php
require("/usr/lib/php/bridge/bridgeclient.class.php");
$client = new bridgeclient();
print("Sending mailbox message 'Hello world from PHP!'\n");
$client->mailbox("Hello world from PHP!");
?>
/usr/bin/php-cli  mailbox.php
Mailbox Read Message

The Mailbox is checked every 10 seconds. The incoming messages will be shown below.

Hello world from PHP!
Waiting 10 seconds before checking the Mailbox again