To save some time here, I believe what I am looking for (!) is the "php switcher" version of this tool rather than a javascript code.

I can't help you with code on this one, but hey check this out: if you are not forcing a font size on your visitors then they can adjust font size with their browser. If they have a vision problem they probably know it eh? Like me for example. So if the font on a web is kinda small for me I increase it via the browser.

Sometimes sites force a font size, meaning I can't increase or decrease it with the browser. Those sites I leave no matter what I wanted from them.

In other words maybe just starting with a reasonably nice size and ensuring that you can increase or decrease via the browser will serve the desired end result without adding code and page hits.

it's doable via javascript or with switching style sheets..
[url=http://skindemo.b2evolution.net/2/index.php?tempskin=grey_skale]grey_skale[/url] skin uses a style swtcher, you'll figure it out.. pretty easy..

Thank you both for your input.

I need to make the site accessible to viewers with visual impairment, one of the tools to use this is the text selector tool. I appreciate that it can be done via the browser but not all users will know how to do that. By having a tool visible on the site, it will make life easier. So, this is the requirement I have and there are no ways around that.

What I am looking for is not the style switcher but a specific font size selector. I have seen some javascript code in it but it requires all the font size attributes in the style sheet to be in "px" and not relative as per the b2 stylesheets which use "em". Here is an example of a js version:

http://www.white-hat-web-design.co.uk/articles/js-fontsize.php

I have done some digging and apparently, there is a php switcher but I'm not sure where to put the code. An example is here:

http://209.85.229.132/search?q=cache:sKvrDkPkRYkJ:www.killersites.com/mvnforum/mvnforum/viewthread%3Fthread%3D7390+text+size+selector+AAA&cd=1&hl=en&ct=clnk&gl=uk&client=firefox-a

Any help would be appreciated.

Can somebody resurrect this enquiry? I am looking for exactly the same thing.

@GSOnline
One of the best sources of information on this subject is...
http://www.456bereastreet.com/

Dive into it as Roger Johansson is one of the best in the world on the subject.
His links are invaluable and his solutions and links are a gold mine.
Example.....
http://www.456bereastreet.com/archive/200909/readable_another_way_of_improving_readability_on_the_web/

BTW. EdB is correct, people are very good at using their browser to solve usability issues like font size.
There is also great tools like....
http://readable-app.appspot.com/
and
http://lab.arc90.com/experiments/readability/

Right, I've cracked it!

Found an absolutely fantastic bit of code that enables font-size changing using PHP and CSS (style switching):

http://www.builderau.com.au/program/php/soa/Text-size-switching-with-PHP-and-CSS/0,339028448,339225202,00.htm

Note: if you use this, the header code will need to go into your _html_header.inc.php file

Oh, and you can use this in conjunction with your existing style.css (get to the end of the instructions and you'll see what I mean).

:)

PS - Thanks @John for the additional info - will certainly be using it as reference.