Here I'm going to show you some absolute paths in PHP and Drupal that you might need.

Absolute path to website root
realpath( '.' )

Absolute path to files directory in Drupal
realpath( file_directory_path() )

Absolute path to current directory (from which the scipt is being run)
realpath( dirname(__FILE__) )

Absolute path to the systems temporary directory (PHP 5)
realpath(sys_get_temp_dir())