The behaviour of these functions is affected by settings in php.ini.
Here's a short explanation of the configuration directives.
Whether or not to enable inclued.
Location (path) to the directory that stores inclued files. If set, each PHP request will create a file. These files are serialized versions of what inclued_get_data() would produce, so may be unserialized with unserialize().
Because every request creates a file, this directory may fill up fast!