Well, you could do a select on the mysql table using the builtin today's date and time function called NOW()
SELECT * FROM datetable_name_here
WHERE
NOW() > start_date_field_name
AND
NOW() < end_date_field_name
If you get any rows, then it is between those dates. try this on the sql command line first, before using php.
You may need to use the to_days function to convert these dates to "number of dates between" to make this sql work.
ex:
to_days(NOW()) - to_days(start_date_field_name) > 0
hope this gets you going in the right direction