You may hear about many outdated issues with SQL and it's use by PHP which no longer matter if you have an up-to-date server but it is best to be aware of the ones explained here.  Just remember that your site can never be completely secure.  Don't keep any data on a site which you don't want taken.  You can make it hard to take a site but in the end they could just steal the physical server if they want it badly enough.