it would probably be easier if you let php determine the starting and ending dates for the date range you want
your target code will look like this, to retrieve all rows for April 2010 --
WHERE s_in >= '2010-04-01'
AND s_in < '2010-05-01'
note that if you use MONTH, MONTHNAME, or YEAR in your WHERE clause, your query will be inefficient
only the range test as shown on the full date is efficient