function get_connection(){
$username= '';
$password = '';
...
return mysqli_connect();
}

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;
}