The ternary (or conditional) operator should do the trick. Something like this:
echo "<tr align='center' class='" . ($selected ? "selected_row" : "red_bg") . "'>";
It's a matter of personal preference whether this is more readable than the original version or not. Keep in mind that code readability and supportability is vastly more important than the "number of bytes" used by the code file.
If this is really happening a lot in the code then there's a fairly decent chance that you could make the code far more clean and expressive by using some kind of templating system to bind page markup to a model or by otherwise refactoring the page rendering into more generic and re-usable modules.