Here's a simple way to select all birthdays for the current date, where the date of birth is stored in a MySQL database in a DATE format, in this examples a column called 'dob' is used.

SELECT username FROM users WHERE MONTH(dob) = MONTH(NOW()) AND DAY(dob) = DAY(NOW())

This will select all users that have a date of birth matching the current month and day the year is ignored.