I`m currently developing WordPress plugins on several servers and I noticed that on some configurations you can`t use $_SESSIONS in your plugins, so I though it will be a good part of our wordpress tutorials section to write about fixing this problem. The SESSION gets killed or it is just not starting. So I started digging through the core to find a solution and guess what I finally did. The problem is related to php.ini. If register_globals = On in php configuration file you have to comment “wp_unregister_GLOBALS();” in wp-settings.php
The WordPress Codex clearly states that if “register_globals=On”,  “wp_unregister_GLOBALS()” will destroy php sessions.

http://codex.wordpress.org/Function_Reference/wp_unregister_GLOBALS

Once again:
To enable php sessions edit wp-settings.php (In your wordpress root), and comment wp_unregister_GLOBALS() on Line:40 after that your code should look like this:

<?php
/**
 * Used to set up and fix common variables and include
 * the WordPress procedural and class library.
 *
 * Allows for some configuration in wp-config.php (see default-constants.php)
 *
 * @internal This file must be parsable by PHP4.
 *
 * @package WordPress
 */

/**
 * Stores the location of the WordPress directory of functions, classes, and core content.
 *
 * @since 1.0.0
 */
define( 'WPINC', 'wp-includes' );

// Include files required for initialization.
require( ABSPATH . WPINC . '/load.php' );
require( ABSPATH . WPINC . '/default-constants.php' );
require( ABSPATH . WPINC . '/version.php' );
require( ABSPATH . WPINC . '/extra_function.php' );
// Set initial default constants including WP_MEMORY_LIMIT, WP_MAX_MEMORY_LIMIT, WP_DEBUG, WP_CONTENT_DIR and WP_CACHE.
wp_initial_constants( );

// Check for the required PHP version and for the MySQL extension or a database drop-in.
wp_check_php_mysql_versions();

// Disable magic quotes at runtime. Magic quotes are added using wpdb later in wp-settings.php.
@ini_set( 'magic_quotes_runtime', 0 );
@ini_set( 'magic_quotes_sybase', 0 );

// Set default timezone in PHP 5.
if ( function_exists( 'date_default_timezone_set' ) )
 date_default_timezone_set( 'UTC' );

// Turn register_globals off.
//wp_unregister_GLOBALS();

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