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 Debugging Tools For Horde 4 or later
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 Horde::debug() 
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To easily debug the value of a variable in Horde PHP code, Horde::debug() can be used.  Usage:

::

 Horde::debug($variable);

$variable is the variable you wish to debug.  The contents of this variable will be dumped into a file named horde_debug.txt in your temporary directory (generally set in horde/config/conf.php in the $conf['tmpdir'] variable).  $variable MUST NOT be debug_backtrace().

If you want to manually specify the location of the debug file, you can use the optional second parameter to Horde::debug():

::

 Horde::debug($variable, $filename);

$filename should be the full path to the filename.  If using $filename, make sure the directory/file is writable by the user the PHP process runs as.

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 IMP: IMAP/POP3 Debugging
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To enable debugging, see instructions contained in imp/config/backends.php (the 'debug' config parameter).

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 General Debugging Tools
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There are many general purpose debugging tools that can be used to debug Horde code. Tools that the developers include, but are not limited to:

* `Xdebug`_: The standard debugging extension for PHP. Beside a few developer niceties, it allows for runtime debugging with breakpoint, step-by-step debugging, introspection, etc. Goes along with browser and IDE extensions like:

  * Emacs extension "geben": http://code.google.com/p/geben-on-emacs/

  * Firefox extension "Easy Xdebug": http://www.elime.be/easyxdebug.htm

* `Wireshark`_: Especially useful for debugging HTTP-based request to and from the Horde code.