Extract the library to a safe place outside your web-directory (your provider should allow this).
Create a .htaccess file to place it in the root directory of your webserver which contains the following line:
php_value include_path ".:/path_to/devcon_php_base"
This allows all php scripts in your webserver to include library files without need to know the particular path where you extracted them.
On a UNIX box the installation process could look like this:
cd /web/www.mydomain.com/ tar xvzf ~/devcon_php_base.tar.gz # Will create directory devcon_php_base/ echo php_value include_path \".:`pwd`/devcon_php_base\" >>htdocs/.htaccess