Quotes mismatch leads to print "$cid" instead of the value of $cid

While trying to fix MDL-14336 I came across nasty code within mod/data/lib.php that uses a mismatch of single and double quotes like this:

echo '<option value="$cid">'.$currentfield->name.'</option>';

which led not to solve $cid into its value, but printing out "$cid" to the HTML source instead. I have cleaned up the whole quotes related code within if (!empty($currentfields) && !empty($newfields)) { } and also added linebreaks after option tags to prevent making kilometers long lines of HTML code. Now the resulting table also displays properly, previously it got cut down somehow.