Error connecting to database - check your database connection properties in constants.php file

SQLSTATE[HY000] [2006] MySQL server has gone away
PHP Version : 7.1.33

( ! ) Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/blackh46/public_html/blackhatlinks/members/admin/includes/Database.php:17) in /home/blackh46/public_html/blackhatlinks/members/admin/includes/Session.php on line 66
Call Stack
#TimeMemoryFunctionLocation
10.0044349328{main}( ).../order.php:0
20.0058349912include( '/home/blackh46/public_html/blackhatlinks/members/admin/includes/controller.php' ).../order.php:2
32.7142376168Session->__construct( ).../controller.php:28
42.7182382296Session->startSession( ).../Session.php:37
52.7182382296session_start ( ).../Session.php:66

( ! ) Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/blackh46/public_html/blackhatlinks/members/admin/includes/Database.php:17) in /home/blackh46/public_html/blackhatlinks/members/admin/includes/Session.php on line 66
Call Stack
#TimeMemoryFunctionLocation
10.0044349328{main}( ).../order.php:0
20.0058349912include( '/home/blackh46/public_html/blackhatlinks/members/admin/includes/controller.php' ).../order.php:2
32.7142376168Session->__construct( ).../controller.php:28
42.7182382296Session->startSession( ).../Session.php:37
52.7182382296session_start ( ).../Session.php:66

( ! ) Warning: PDO::prepare(): SQLSTATE[00000]: No error: PDO constructor was not called in /home/blackh46/public_html/blackhatlinks/members/admin/includes/Configs.php on line 15
Call Stack
#TimeMemoryFunctionLocation
10.0044349328{main}( ).../order.php:0
20.0058349912include( '/home/blackh46/public_html/blackhatlinks/members/admin/includes/controller.php' ).../order.php:2
32.7142376168Session->__construct( ).../controller.php:28
42.7182382296Session->startSession( ).../Session.php:37
52.7260384520Session->addActiveGuest( ).../Session.php:78
62.7260384520Configs->getConfig( ).../Session.php:542
72.7260384520prepare ( ).../Configs.php:15

( ! ) Fatal error: Uncaught Error: Call to a member function execute() on null in /home/blackh46/public_html/blackhatlinks/members/admin/includes/Configs.php on line 16
( ! ) Error: Call to a member function execute() on null in /home/blackh46/public_html/blackhatlinks/members/admin/includes/Configs.php on line 16
Call Stack
#TimeMemoryFunctionLocation
10.0044349328{main}( ).../order.php:0
20.0058349912include( '/home/blackh46/public_html/blackhatlinks/members/admin/includes/controller.php' ).../order.php:2
32.7142376168Session->__construct( ).../controller.php:28
42.7182382296Session->startSession( ).../Session.php:37
52.7260384520Session->addActiveGuest( ).../Session.php:78
62.7260384520Configs->getConfig( ).../Session.php:542