This patch allows
php-eval to be used with php code from stdin.
echo 'print "hello world!\n";' | drush php-eval -
The arg to php-eval should be '-' to indicate that the php code should be read from stdin. This could have been a test to check for an empty command, but then drush would hang waiting for input if you ran php-eval with no parameters.
There is an existing unix standard that - in place of a file means stdin; maybe this patch should have modified php-script instead, but I think this is okay as well.
The patch still needs documentation. I'm sketchy on the value of this, so someone who cares more about it will have to finish it up. For the record, I was working on this to investigate.