How to initiate a prog. via mysql

[abhishek jain]

Dear Friends,
I need to start a prog. as soon as a particular field in the mySQL 4.x is
changed. ie. when in Flag table if_flag is set to '1'  i need to start a
script.
The script will be in PHP.
Pl. tell me how to do so on mySQL 4.x.
I shall be very grateful.
--
Regards,
Abhishek Jain