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Serial port access from PHP with Raspberry Pi

adapted from:

One very interesting type of application of the Raspberry Pi is to control our environment, typically some sort of home automation or monitoring device. For that, you need to be able to control and/or monitor hardware devices, like relays, temperature sensors and the like. The Raspberry Pi is not well adapted to directly controlling hardware because of its limited IOs (it does not have analog inputs or outputs for instance). Also, let's face it, in spite of the processing power of the CPU in the Raspberry PI, some tasks are simply more easily done with a small microcontroller like an Arduino or an 8051 (put your favorite microcontroller here...)

This page will show you how to interface a web page served from a Raspberry Pi to an Arduino via the serial port.

Installing PHP-Serial

Download it from Github:

Configuring your Raspberry Pi for PHP serial class

Before you can use the class some stuff must happen (I assume you have Apache installed and PHP enabled and the Raspberry Pi is running). First we need to find out what user runs PHP. Create a file named whoami.php under the /var/www directory as follows:

root@raspberrypi:/var/www# nano whoami.php

Add the following content:

echo exec('whoami');

Then fetch the whoami.php page from your laptop or PC's web browser.

For Apache on Raspbian, the browser will return www-data. It may be different with other web servers.

Every serial connection (virtual of physical) is owned by the dialout group, so if we add www-data to the dialout group our PHP scripts should be able to open/read/write the serial device, which is better than running everything as root, or setting permissions to "all can read from and write to everything". The following command will add the dialout group to www-data.

root@raspberrypi:/opt/www# usermod -a -G dialout www-data

Running the command groups www-data give the following result:

root@raspberrypi:/opt/www# groups www-data
www-data : www-data dialout

Great, www-data belongs to dialout and www-data. Now RESTART your Raspberry Pi. 

root@raspberrypi:/opt/www# reboot

Testing the connection

To test the PHP setup, I’ll write a simple script that just sends a string to the Arduino.


ini_set('display_errors', '1');
include "php_serial.class.php";

$serial = new phpSerial;
$serial->sendMessage("Hello from my PHP script, say hi back!");


echo "I've sended a message! \n\r";

First I enable all the errors, the PHP_Serial class issues warnings on failure and by default they aren’t displayed (in a normal PHP configuration). Then I include the PHP_Serial class file. Next I initiate a new phpSerial object called $serial and configure some parameters. We don’t have parity, characters are 8 bits and we use 1 stop bit. After that I can open the device send my message and *close it*. Finally I echo some feedback to the browser saying I did my job.

Note: It is very important to close the serial device each time, otherwise it will stay open and the script will only work once until your reboot.

I don’t know how or why but for every connection I open, I get question marks (unknown chars). They have a decimal value of 254 and I really don’t have a clue what they are. When I use a normal echo command in the terminal I don’t get those characters.

Debugging the PHP serial class

A lot can go wrong, so lets cover the basics.

If your browser keeps loading and nothing happens then your Serial connection is locked up, restart your Pi to release it and see that you close the device in your PHP script.

If that does not fix it then there might be a problem with user permissions. Go back to the first part of this page to make user that the user www-data belongs to the dialout group. Use the following command to check if the dialout group has access to the /dev/ttyAMA0 device:

root@raspberrypi:/opt/www# ls -l /dev/ttyAMA0
crw-rw---T 1 root dialout 204, 64 Aug 30 20:21 /dev/ttyAMA0

If the browser says the message is sent but you don’t see anything on the Arduino serial monitor then check for common flaws: unplugged cables, wrong level converter circuit, baud rate and so on.