If your PHP Library is a class or set of classes, you can add them to the registry. This is what I am doing in the Mobile Detect module. I got the idea from another module, but I don't recall which one:
/**
* Implements hook_registry_files_alter().
*
* Adds the Mobile_Detect() class to the registry so that autoloading works.
* See http://drupal.stackexchange.com/questions/42266 for a discussion.
*/
function mobile_detect_registry_files_alter(&$files, $modules) {
$library_path = _mobile_detect_get_library_path();
$files[$library_path . '/Mobile_Detect.php'] = array(
'module' => 'mobile_detect',
'weight' => 0,
);
}
This adds the class to registry so it will be autoloaded if instantiated as $detect = new Mobile_Detect();
. You can edit the $library_path = ...
line to suit your situation.