A new language construct as a 'friend' to echo wouldn't be a bad idea IMHO. Something like echoerr which prints to stderr instead of stdout (and does not use output buffering of course).

The current situation requires the following steps:

$f = fopen('php://stderr', 'w');
fputs($f, 'debug message');
close($f);

which is ... well, it's nice to have but cumbersome to use (obviously, I hope).

Example:
echo "ene", "mene", str_repeat('bla', '3), "\n";

should just work with echoerr the same:

echoerr "ene", "mene", str_repeat('bla', '3), "\n";

The reason I'm for a language construct:
Make it as similar to use as echo itself. Not requireing it having to use parentheses. Easily to replaceable echo and echoerr.

That's it, flames on.