Sets the value of the given configuration option. Returns the old
value on success, FALSE on failure. The configuration option
will keep this new value during the script's execution, and will
be restored at the script's ending.
Not all the available options can be changed using ini_set(). There is a list of all available options in the appendix.
See also: get_cfg_var(), ini_get(), ini_get_all(), ini_restore() 和 How to change configuration settings