MYSQL SELECT WHERE IN issue

I am trying to use the WHERE fieldname IN ("#form.names#") but it won't work.

I know for example:

SELECT *
FROM tablename
WHERE
fieldname IN ('US,UK,GB,CN');

will NOT work, but

SELECT *
FROM tablename
WHERE
fieldname IN ('US','UK','GB','CN');

will work.

But here is my problem.  I get the list from a form with checkboxes, so they are stored in a form variable call form.names.  The form.names value is US.UK,GB,CN etc.  So if I put

SELECT *
FROM tablename
WHERE
fieldname IN (#form.names#);

nothing will work, because there is no quotes around it but if I put the quotes

SELECT *
FROM tablename
WHERE
fieldname IN ("#form.names#");

will work ONLY if the list has one item.  When there is more than one like the above, it won't work at all.

How can I manipulate the #form.names# so that each item will have quotes around it?  Any help is appreciated.