Change PHP version on server using either .htaccess or php.ini
Question
How can I change the PHP version to 5.2 on a server using either an .htaccess of php.ini file?
Solution
Adding
AddHandler application/x-httpd-php52 .php .php5 .php4 .php3
to your .htaccess might work.
OTHER TIPS
I take it you want to process e.g. .php3 files with php 3.x.
You can't switch between php versions like this, but you might be able to do it by setting up a proxy server that distributes the query to one of several servers (one for each PHP version).
There might also be some apache hack that works by calling the cgi-bin mode php in FastCGI mode. See the linked answer for this.
According to https://www.first2host.co.uk/f/change-php-version-php-ini/, adding the following line to your php.ini file will change you to PHP version 5.2:
AddHandler application/x-httpd-php52 .php
It only works if your host is first2host though, I reckon.