Implement a custom PHP library into a custom block module? I have a PHP class and it's files in the folder:

/var/www/include_php/**the_folder**


How should I include this into Drupal?

ln -sf /var/www/include_php/the_folder /var/www/drupalSite/sites/all/libraries/the_folder


and then do drupal_add_library('the_folder') or something else in my block module?
drupal_add_library() is for Javascript/CSS libraries; Drupal doesn't have a similar registry for PHP libraries/classes.

To include a file inside a module in your installation, use module_load_include() e.g.

module_load_include('inc', 'mymodule', 'mymodule.pages');


For all others just defer to the standard PHP functions (include/require, etc.)