Run a script before each page

Hello,

I want to run a php-script before any of my webpages load. I know that I put
a simple line in the beginning of each page, but that's not what I want. I'm
thinking of a line in the .htaccess file or whatever, so that I can't forget
to put a line before each page. This idea is for a logfile.

I hope that I explained my question clearly (in my best English).
If not, let me know...

Thanks in advance!

Jan (from Holland)