I suspect that anytime the query doesn't return a row, you're going to get that UNDEFINED VARIABLE. We don't see that the variable is defined/initialized anywhere except after a row is fetched.
As far as why the query doesn't return a row, your query is equivalent to
... WHERE stuff = 'food, drink, cooking'
That's a single string literal. The query is only going to return rows were the column stuff is equal to that string. The commas inside that string are just part of the value. If you want to find rows where stuff contains any of the values, your query would need to be of the form:
... WHERE stuff IN ('food','drink','cooking')
Note that this is three separate string literals, separated by commas. The commas are part of the SQL statement, not part of the value. This would return rows where the column stuff contains 'food'. Or 'drink'. Or 'cooking'.
That's why your query "doesn't work".
