Constants of wp-config.php

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Currently there are several PHP constants on the wp-config.php that will allow you to improve you WordPress code and help you debug.
WP_DEBUG
This is an Option included in WordPress version 2.3.1.
By default this will be set to false which will prevent warnings and errors from been shown, but all WordPress developers should have this option active.
Activates the Logs
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define( 'WP_DEBUG', true );
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Deactivates the Logs
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define( 'WP_DEBUG', false );
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Check that the values must be bool instead of string
A minor patch later the on Wordpress version 2.3.2, the system allowed us to have a more granular control over the Database error logs.
Later on in the version 2.5, WordPress raised the error reporting level to E_ALL, that allows us to see logs for Notices and Deprecation messages.
Notes:
If you have this option turned on, you might encounter problems with AJAX requests. This problem is related to Notices being printed on the output of the AJAX response, that will break XML and JSON.
WP_DEBUG_LOG
When you use WP_DEBUG set to true you have access to this constant, and this will allow you to log your notices and warnings to a file.
WP_DEBUG_DISPLAY
When you use WP_DEBUG set to true you have access to this constant; with it you can choose to display or not the notices and warnings on the screen.
Note:
If these variables don't produce the output you are expecting check out the Codex Section about ways to setup your logging.
SCRIPT_DEBUG
When you have a WordPress plugin or theme that is including the Minified version of your CSS or JavaScript files by default you are doing it wrong!
Following the WordPress idea of creating a file for development and its minified version is very good and you should have both files in your plugin, and based on this variable you will enqueue one or the other.
By default this constant will be set to false, and if you want to be able to debug CSS or JavaScript files from WordPress you should turn it to true.
Activates the Logs
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define( 'SCRIPT_DEBUG', true );
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Check that the values must be bool instead of string
WordPress default files wp-includes and wp-admin will be set to its development version if set to true.
CONCATENATE_SCRIPTS
On your WordPress administration you will have all your JavaScript files concatenated in to one single request based on the dependencies and priority of enqueue.
To remove this feature all around you can set this constant to false.
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define( 'CONCATENATE_SCRIPTS', false );
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SAVEQUERIES
When you are dealing with the database you might want to save your queries so that you can debug what is happening inside of your plugin or theme.
Make $wpdb save Queries
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define( 'SAVEQUERIES', true );
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Note: this will slowdown your WordPress