Open database persistently

resource dba_popen ( string $path , string $mode [, string $handler [, mixed $... ]] )

dba_popen() establishes a persistent database instance for path with mode using handler.

path: Commonly a regular path in your filesystem.
mode: It is r for read access, w for read/write access to an already existing database, c for read/write access and database creation if it doesn't currently exist, and n for create, truncate and read/write access.
handler: The name of the handler which shall be used for accessing path. It is passed all optional parameters given to dba_popen() and can act on behalf of them.

Returns a positive handle on success 或者在失败时返回 FALSE.