If you are running PHPUnit and Selenium though the command line you can use the STDIN Example found on PHP.net's Input/output streams page
$line = trim(fgets(STDIN));
when the code reaches that lime everything will pause and wait until you press Enter, so if you use code like this
echo " Command: "; $line = trim(fgets(STDIN)); echo " Input = '".$line ."'";
you can then get text entered into the command line that you can then use selenium to input that into the CAPTCHA form.
the only problem is that if you let Selenium idle too long the session till time out and thus cause errors when the code resumes (and currently looking to try and get around this)