phpa is an interactive command line shell for PHP.
It is a replacement for the interactive mode of the PHP interpreter (php -a), hence the name.
After starting phpa, you will see the >>> prompt. You can enter nearly any PHP code here.
phpa tries to be smart about the code you give it. If the code looks like an immediate (i.e. doesn't contain assignments, blocks, control structures, etc.) and doesn't produce any output, then the result of the code is printed. This makes it easy to test various things without unnecessary typing.
A newline is automatically added to the end of any output that doesn't already end with one.
phpa supports tab completion for functions and constants. This works like filename completion in the bash shell. Start typing the name of a function or constant then press the tab key to complete the name or see a list of possible matches.
Alternatives
PHP Interactive is a web based interactive shell for PHP - http://www.hping.org/phpinteractive/
PHP Shell is similar to phpa, but takes an object-oriented approach and has more features, including fatal error handing - http://jan.kneschke.de/projects/php-shell/