Fixed E_NOTICE in boot.php and DBA class (#5430)

Fixed E_NOTICE in boot.php and DBA class (#5430)

* Fixes:
- fixed missing variable $port (MySQL: 3306)
- "imported" mysqli class

Signed-off-by: Roland Häder <roland@mxchange.org>
* Fixed:
- better use `false` and `$port > 0`

* And better only provide `$port` when larger zero.

* Initialize `$port` with zero value (int) and not `false` (bool).

* Removed duplicate mysqli "import".

* `$post_update` is no longer used. Instead `$prefix` needs to be checked.