While learning php I found this common statement
$add_tel_sql = "INSERT INTO telephone (master_id, date_added, date_modified, tel_number, type) VALUES ('".$master_id."', now(), now(), '".$safe_tel_number."', '".$_POST['tel_type']."')";
What I do not understand why in the '".$master_id."' we are using the " and also the dot. why we do not only use $master_id without the dots and the quotation marks. I would appreciate any explanation.