Automattic

Donncha O Caoimh (a11n)

This plugin logs the activity of supported plugins to a file, making it easier to figure out why there’s a problem.
If you have been asked to install this plugin because of a support issue then installing this plugin is just like installing any other plugin. Once installed, go to Settings->WP Logger and enable logging. If you have a number of supported plugins enable the one you are interested in.
Developers who want to use this plugin to add logging to their own plugin should see the Developers section of this documentation.
Mark Jaquith’s <a href="https://wordpress.org/extend/plugins/monitor-pages/" rel="ugc">Monitor Pages</a> plugin was used as a template for this plugin. Thanks Mark!
<h4>Developers</h4>
Other plugins can use this plugin to record important events which are then dumped to a log file. Plugins must add the event to a global array in the following way:
<pre><code>$GLOBALS[ 'wp_log' ][ 'name_of_plugin' ][] = 'Some important event';
</code></pre>
Plugins must also add themselves to a list of enabled plugins. This will make it easier for blog owners to filter out which plugin they want to debug on the settings page. Add and edit the following code so it is executed when the plugin is loaded.
<pre><code>$GLOBALS[ 'wp_log_plugins' ][] = 'name_of_plugin';
</code></pre>
A hypothetical example might be a plugin (let’s call it “Big A”) that uses the output buffer to change the letter “a” to “A”. The events recorded by this might include the following:
<pre><code>$GLOBALS[ 'wp_log' ][ 'big_a' ][] = 'Created output buffer';
$GLOBALS[ 'wp_log' ][ 'big_a' ][] = 'Output buffer callback';
$GLOBALS[ 'wp_log' ][ 'big_a' ][] = 'Replaced a with A in page';
</code></pre>
This plugin takes the “wp_log” array and dumps it to a file in the upload directory of the blog it’s activated on. The file is linked from the plugin settings page and can be deleted on that page too. The file is a simple text file and is not protected in any way so be careful if auth cookies are logged by a plugin.
A “log” function isn’t included but if you want to add one to your plugin check the constant WP_DEBUG_LOG is defined before adding to the log array. If your plugin doesn’t use a class make sure that you call the log function a unique name so it doesn’t conflict with other plugins.
<pre><code>function log( $message ) {
 if ( defined( 'WP_DEBUG_LOG' ) )
 $GLOBALS[ 'wp_log' ][ 'name_of_plugin' ][] = $message;
}
</code></pre>

WP Debug Logger

A plugin that makes it easy to log code activity to a file.

chandima

Display log messages from PHP in the browser console in Safari, Firefox (with firebug), and Opera (with the new Dragonfly debugging environment). Essential debugging tool for 
plugin and theme developers. You no longer have to use ‘print_r’ statements from PHP to figure out what is going 
on in the code, which more often than not, messes up the DOM and HTML layout. Displays complex PHP structures like arrays and objects 
in pretty print.
Features
<ul>
<li>Log debug messages directly from themes and plugins.</li>
<li>Display log messages in the browser console, without muddying up the browser display.</li>
<li>Displays complex structures such as arrays and objects in pretty print.</li>
<li>Shows the line number and file from where the message was logged ( you won’t lose track of log statements ).</li>
</ul>
For more info, comments, and feature requests, visit <a href="http://www.turingtarpit.com/2009/05/wplogger" title="Wordpress Logger" rel="nofollow ugc">the plugin homepage</a>.
<h3>Credits</h3>
Code that forces the wplogger plugin to load first was adapted from the WordPress 
<a href="http://inchoo.net/wordpress/wordpress-firephp-plugin/" title="FirePHP plugin" rel="nofollow ugc">FirePHP plugin</a> 
developed by Ivan Weiller.
This plugin is based on <a href="http://pear.php.net/package/Log" title="PEAR Log" rel="nofollow ugc">PEAR Log</a>, the logging framework that is part of the 
PHP PEAR library. Current maintainers Jon Parise, Jan Schneider, and Chuck Hagenbuch. PEAR Log is based on code first 
developed for the Horde 1.3 framework – original authors Chuck Hagenbuch, and Jon Parise.
<h3>Release Notes</h3>
0.3 : Added support for Opera ;
0.2.2 : Fixed formatting issues in readme.txt file;
0.2.1 : Added more detail to the readme.txt file;
0.2 : First official public release;
0.1 : First internal release;

WordPress Logger

Display log messages during plugin and theme development on the console in Safari and Firefox (with Firebug) browsers.

WP Debug Logger vs WordPress Logger