Tutorial ideas wanted

[cwarn23]

Please post a list of ideas for tutorials that can be written for the php section. Please bear in mind the tutorials are to be kept relatively small so don't ask for something that may take pages upon pages of text to explain. Thanks.

OK, here are some of the problems I see most often:

login/registration
post handling for multiple checkboxes with same 'name' attribute
how to use header redirect!
emailing (with an attachment)
working with dates (subtracting and adding days etc) to get new dates
use of regex in preg_* functions - users often don't realise that these are the functions that they require

[pritaeas]

One that comes up a lot, is two dropdowns with linked info (although it involves javascript). I was planning on writing one, but still haven't. I do have a working example though. If you want it I can zip it (uses PHP, MySQL and jQuery).

One thing popping up in the forum lately, is how to handle notices for undefined indexes and variables (using the isset construct). A sticky for that to refer to would be nice as well.

[cwarn23]

I've just submitted a redirect tutorial and am waiting for staff writers to approve. Soon I'll have dinner and start on the next tutorial.

Just remembered another one:

when using a loop, many users do a query on every iteration, whereas they could just build up a values or where clause and send just the one query at the end of the loop. I've answered a few of these lately.

//edit

perhaps something on general security, e.g.
preventing sql injection,
XSS,
sessions,
location of include/config files etc