Just a note for people trying to use interactive mode from the commandline.
The purpose of interactive mode is to parse code snippits without actually leaving php, and it works like this:
[root@localhost php-4.3.4]# php -a
Interactive mode enabled
<?php echo "hi!"; ?>
<note, here we would press CTRL-D to parse everything we've entered so far>
hi!
<?php exit(); ?>
<ctrl-d here again>
[root@localhost php-4.3.4]#
I noticed this somehow got ommited from the docs, hope it helps someone!