It appears that if you use both the 'include_path' directives and 'open_basedir', that file searches will hit the include path *first*, before local files. But if 'open_basedir' is not in use, then local files are found first. For example, suppose you have code in '/var/www/myfile.php' which does:
Further, assume that there is a local file '/var/www/config.php', and there is also a file '/var/local/php/config.php'.
Next, if your php.ini has:
include_path = /var/local/php/
Normally, this would look for '/var/www/config.php' first, and if not found, then it would try '/var/local/php/config.php'.
But if you also have this in php.ini:
open_basedir = /var/www/:/var/local/php/
Then the require would reverse the order of the search, and load '/var/local/php/config.php', even when the local 'config.php' file exists.
Furthermore, if include_path contains directories not in open_basedir, you can end up with a fatal error. For example, change the directive to:
open_basedir = /var/www/:/var/local/includes/php/
Now the require will first find '/var/local/php/config.php' from the include_path, try to include it, but be unable to because of the open_basedir restrictions.