Try using the sql_port value as a number rather than a string.
And, if you want to clean up your conf_global.php file, these are the values you'd have to keep comparing it to a fresh 4.x install (not sure if about your 3 top_site_list rows, some really old mod perhaps?):
<?php
$INFO = array (
'sql_host' => '127.0.0.1',
'sql_database' => 'abcdefg',
'sql_user' => 'abcdefg',
'sql_pass' => '12345678',
'sql_port' => 3309,
'sql_tbl_prefix' => 'ddrr_',
'sql_utf8mb4' => true,
'board_start' => '1131793624',
'installed' => true,
'base_url' => 'https://www.thesecurityinstaller.co.uk/community/',
'admin_group' => '4',
'guest_group' => '2',
'member_group' => '3',
);