function get_connection(){
static $connection = null;

if($connection){ return $connection; }
$username= '';
$password = '';
...

$connection = mysqli_connect();

return $connection
}

function get_tests(){
$CONNECT = get_connection();
$res = mysqli_query($CONNECT, "SELECT * FROM `test`");
if(!$res) return;
$data = array();
while($row = mysqli_fetch_assoc($res)){
$data[] = $row;
}
return $data;
}