PHP 3.0.13 and up, php://output since PHP 4.3
php://stdin
php://stdout
php://stderr
php://output
php://stdin, php://stdout and php://stderr allow access to the corresponding input or output stream of the PHP process.
php://output allows you to write to the output buffer mechanism in the same way as print() and echo().
php://stdin is read-only, whereas php://stdout, php://stderr and php://output are write-only.