I've decided to use PHP 4.1.1 and MySql to develop my site running on Tomcat using Microsoft-IIS/6.0, the latter details being meaningless to me as i haven't a clue what that does or doesn't mean. Also, i'm using ADOdb to talk to MySql and any existing customer database ii may have to deal with in the future.
Anyway, i have no real problems with the coding, i've taken a procedural approach despite this version of PHP supporting modularisation. Although my coding is primitive by experienced developers standards it is suprisingly brief and functions adequately.
My concern now is how and where my set-up will deal with multiple concurrent user requests which i assume is thread handling. I'm also concerned about bottlenecking of the database and Tomcats capabilities (would Apache be better). Finally, if by some miracle anything i produce needs scaling what support does my set provide. Excuse the volume of questions but i'm a bit of a novice in at the deep end.

I would strongly advise you to update your version of PHP unless you are happy configuring your php.ini file for aspects such as greater security. Even then I would say upgrade to the latest version of 4. I would also advise you to use the mysql functions native to version 4 when connecting to a mysql database. These are well-documented in the PHP manual

You may consider updating to version 5 although this may not have any advantages for you depending on what you are doing, also the mysql support is not as seamless as it is in version 4.

There is nothing inherently wrong with the procedural approach. If your applications aren't large, stick with it.

The Apache, MySQL, PHP combination has excellent scaling potential and is very well-supported within the community.

PHP usually runs as a compiled in module for Apache making it significantly faster than as a cgi handler. Not sure how it works with Tomcat.