Setting include path in PHP intermittently fails. Why?

ini_set('include_path', '.:/usr/share/php5:/usr/share/php5/PEAR:lib:app/classes');

php_value include_path ".:/usr/share/php5:/usr/share/php5/PEAR:lib:app/classes"

ini_set('include_path', ini_get('include_path') . PATH_SEPARATOR . 'lib' . PATH_SEPARATOR . 'app' . DIRECTORY_SEPARATOR . 'classes');

ini_get('include_path')

set_include_path('.' . PATH_SEPARATOR . './app/lib' . PATH_SEPARATOR . get_include_path());