error_log("***** PHP -> " . $myVar, 0);
Open a console — iterm is great if you want an excellent terminal on OSX — and issue the following command. Keep in mind if your error_log file is in a different location you will need to find where your Apache installation has placed it. The tail command is perfect for monitoring the log messages as your app dumps variables. Grep will filter out just the PHP lines in your log making it easy to follow your app messages.
tail -f /var/log/apache2/error_log | grep PHP