Thanks @boomoto for helping me figuring out the answer:
$query = "UPDATE tableA ..
$response = $dbc->query($query);
if(mysqli_affected_rows($dbc)==0){
$query2 = "INSERT INTO tableB ..
$response2 = $dbc->query($query2);
$query = "INSERT INTO tableA ..
$response = $dbc->query($query);
}
}