To debug WordPress many sites will tell you to add the WP_DEBUG setting on its own to your wp-config.php file, but this can be quite harmful as many server configurations will start showing PHP errors and warning messages on your site pages. Most sites will also log any errors to some form or error log, but this isn't always the best place for them to go.
XDebug is an excellent debugging solution that will give you a more detailed indication of what is wrong in your code. Here is an example of trying to divide two variables that have a value of 0.
If you are debugging a PHP application then you might want more information than the values of some current variables. There are a number of built in magic variables that can be used to print out the file name, line number, class and method that the debug statement is printed out on. Here is an example that prints out some information from a class.
Krumo is an open source plugin for your programs that is designed as a replacement to print_r() and var_dump(). These functions are used by developers (myself included) to find out what the program is doing. The main problem is that if there is a lot of data to look at the page can get a bit busy.