<?php
$file = fopen("test.txt","w");
fwrite($file,"Hello World. Testing!");
fclose($file);
?>

tail -n 15 /var/log/apache2/error.log

[:error] [pid 7505] [client 127.0.0.1:47066] PHP Warning:  fopen(/test.txt): failed to open stream: Permission denied in /var/www/html/a.php on line 2