For debugging I prefer var_dump()
. If you install Xdebug (http://xdebug.org/) in your development environment, you'll get a lot more out of var_dump.
When I want to kill a script I'm debugging, I wrap die around that, as in die(var_dump())
.
For logging, look into PHP's error_log()
(http://www.php.net/manual/en/function.error-log.php) function or a logging library such as monolog (https://github.com/Seldaek/monolog).