Write hex to serial port from PHP

Question

I'm using a Raspberry PI (Jessi) to send a string of hex characters to a device using the serial port (USB to serial port). I use the following command successfully from the command line:

echo -en "\x63\x69\x72\x62\x0d" > /dev/ttyUSB0

I want to do the same thing using PHP on the same Raspberry Pi but when I try to send the same command from a PHP file I do not get the desired result. Example PHP code:

var_dump( shell_exec('echo -en "\x63\x69\x72\x62\x0d" > /dev/ttyUSB0') );

I have tried a lot of things to get this working:

• I have added www-data (PHP user) to group dialout (group for /dev/ttyUSB0)
• I have set chmod 777 for /dev/ttyUSB0
• I have verified that PHP can write to /dev/ttyUSB0 (is_writable())
• I have tried a lot of different stty settings (ex: stty -F /dev/ttyUSB0 -isig -icanon)
• I have tried different ways in PHP to write to the serial port (PhpSerial library, pack() and fwrite(), printf() etc) Among other things.

I have not found a way to see exactly what PHP is writing to /dev/ttyUSB0 (screen, minicom) but I know it does something since after I have tried the PHP script the next time I try from the command line it does not work. The second time, after trying from PHP, the try from the command line is successful.

I have tried running the PHP file from command line (CLI, both PHP5 and PHP7) and using the web server but nothing works.

I have tried all the things I can think of, and found online, but I can still not get it to work.

What could cause this problem? The command works, I just want to run it using PHP, but it fails to do so. Other command I try, using PHP, works - for example:

var_dump( shell_exec('whoami') );

Update #1 I created a bash script file with the working command and also added an echo "ok" to get some output back. The file works when running it from the command line but when I try to run it from PHP I get the "ok" back but it still fail to effect the connected device. I tried to run the bash script from the command line as the www-data user and that works, but still nothing I do using PHP works:

sudo -u www-data bash -c '/home/test/cmd'

Update #2 Tried the same thing on another computer, running Kodi, with the same negative result. The command works but not when run using PHP.

Answer 1

I ran into a similar problem. What I eventually figured out was that user httpd (I'm running CentOS -- the apache user might be different on your flavor of Linux) didn't have write permissions to /dev/ttyS0.

// Grants write permission on /dev/ttyS0 to all users

sudo chmod a+w /dev/ttyS0

It'd be a bit more elegant to figure out how to grant write permission only to the httpd user instead of all users on the machine.

Hope this helps you out!