environmental-sensor-uart/expirements/test-read.php

<?php

// check if we can write to the uart device file
clearstatcache();
if(false === is_writable("/dev/ttyACM0")) exit("check permissions: /dev/ttyACM0" . PHP_EOL);

$fd = fopen("/dev/ttyACM0", "c+");

// first read  after initial power up:
$first = fread($fd, 1024);

// send an 'R', then read again
fwrite($fd, "R" . PHP_EOL);
sleep(1);
$second = fread($fd, 1024);

fclose($fd);

var_dump(strlen($first), $first, PHP_EOL, "hello", strlen($second), $second);