The CLI SAPI defines a few constants for I/O streams to make programming for the command line a bit easier.
| Constant | Description | 
|---|---|
| STDIN | An already opened stream to stdin. This saves opening it with 
<?php
<?php | 
| STDOUT | An already opened stream to stdout. This saves opening it with 
<?php | 
| STDERR | An already opened stream to stderr. This saves opening it with 
<?php | 
Given the above, you don't need to open e.g. a stream for stderr yourself but simply use the constant instead of the stream resource:
php -r 'fwrite(STDERR, "stderr\n");'
Note:
These constants are not available if reading the PHP script from stdin.