SECURITY PRECAUTION BEFORE SUBMITTING DATA IN DATABASE

Hi, I am designing a php website for my client which interact with database. This is my first project for any client (I hope he is not reading this mail ;-) ). I am a bit more concerned with database security. Can somebody shed some light on the security measurements, precautions, and functions related to database security in general to make sure that the data is safely stored updated and retried from database. I have already used htmlentities(), strip_tags(), addhashes(), and some regular expressions to check security. Looking for help beyond this. Thanks in advance... Sumit