Can be useful to determine whether output is being piped elsewhere, for PHP shell scripts, for example;
#!/usr/local/bin/php
<?php
if ( !posix_isatty(STDOUT) ) {
fwrite(STDOUT, "Invalid TTY\n");
exit(2);
}
fwrite(STDOUT, "Enter you name\n");
$name = fgets(STDIN);
fwrite(STDOUT,"Hello $name\n");
exit(0);
?>
If this script is like:
script.php > outfile
outfile will contain "Invalid TTY"