executing script at specified time

How can I execute a php script at a specified time using an Apache server? Let's say I wanted to send everyone an email, who had updated their personal information within the last 24 hours, every night at midnight... I would write a script that would get the current date and time, check a mysql database for all rows that met the date critieria, and automatically email everyone that met that criteria. I am guessing most administrators would not want to make sure they are in front of a browser at midnight every night to run the script, so how can I do this? :) Thanks ahead of time! Rick