necessary or not

I've always given value to a variable which included the value of
another variable in the following way.

$thanks = "Thank you, ".$fname." ".$lname." for registering.";

In working with MySQL queries I haven't used the concatenation.

For example:

$query = "
INSERT INTO userinfo (fname,lname)
VALUES ('$fname','$lname')
";

OR

$query = "
SELECT fname,lname FROM userinfo WHERE userinfoID='$userid'
";


My question, would it be more correct or better to write the above in
the following way?

$query = "
INSERT INTO userinfo (fname,lname)
VALUES ('".$fname."','".$lname."')
";

OR

$query = "
SELECT fname,lname FROM userinfo WHERE userinfoID='".$userid."'
";



Thanks,
Pasquale