In case you're wondering exactly where and how WordPress stores the information about activated plugins, have a look in the database. Using your MySQL client, you can browse the wp_options
table or execute the following query:
SELECT option_value FROM wp_options WHERE option_name='active_plugins';
The active plugins are stored as a serialized PHP hash, referencing the file containing the header. The following is an example of what the serialized hash might contain if you had activated a plugin named "Bad Example". You can use PHP's unserialize() function to parse the contents of this string into a PHP variable as in the following script:
<?php $active_plugin_str = 'a:1:{i:0;s:27:"bad-example/bad-example.php";}'; print_r( unserialize($active_plugin_str) ); ?>
And here's its output:
Array ( [0] => bad-example/bad-example.php )