Switch and Injection

I’m getting better with my sanitization, thanks to all the help here.

Now I wonder.

I’m combining a muti-page php form into a single (hopefully cleaner) php file. 

Normally I would sanitize and check user rights on every single page. 

(output from the form is dependent on individual rights of the user stored in a DB)

I want to protect against injection, and my gut says I should simply place the sanitization function and user rights verification at the top of the page….but I wonder if it should be placed at the top of every switch case. 

Thoughts?

Thanks All…!

(I’m going to start helping an actual php programmer with a side project of his…I'm sure to learn a lot, so hopefully I’ll be able to stop asking so many questions, and start helping to answer a few.  I appreciate everyone’s generous help!)