get_cfg_var returns the value from php.ini directly,while the ini_get returns the runtime config value. I have tried it on PHP 5.1.6
[EDIT by danbrown AT php DOT net: The author of this note means that ini_get() will return values set by ini_set(), .htaccess, a local php.ini file, and other functions at runtime. Conversely, get_cfg_var() will return strictly the server php.ini.]