What tool are available in the wordpress API for debugging a plugin?

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1. You should NEVER work on a live site

Just setup a local installation of WAMP or MAMP and do your testing there. This has a lot of benefits:

  • You can’t break a live site
  • You don’t need to upload changed files
  • It runs faster as everything is local and doesn’t go trough the internet

2. Debugging

First you could enable debug in config.php

define('WP_DEBUG', true);

This will give you a lot more information what went wrong if something goes wrong.

There also is a lot more info in the WordPress Codex on Debugging in WordPress