The orginal question was regarding a WordPress plugin that needed PHP5 in order to work correctly.  The owner of the WordPress website switched from PHP4 to PHP5 in the web host control panel, but PHP4 version was still being seen as active and PHP5 was not recognized.

Solution for this WordPress PHP5 problem:  The php.ini file did not exist in the root and admin folders.  Once the php.ini files were added PHP5 version was recognized.

Checked your website and I see that your web host has PHP5 available. You have WordPress 2.9.2 installed – Currently WordPress works on both PHP 4 and PHP 5 based platforms so that is not the cause of the PHP5 problem. Your using the Suffusion WordPress Theme – Older Themes and Plugins may have PHP code that will not work with PHP 5. You should check with the Theme authors for any PHP5 issues with your Theme and see if they have updated Theme versions. I didn’t check this out so you should double check to see if their are any issues with the older versions of the Suffusion WordPress Theme.

You most likely already did this – Did you create a phpinfo.php file to double check that you really have PHP5 running on your web host server?

For anyone who may want to know how to do this:
Create a text file called phpinfo.php with the following code below and upload it to the root of your server and run it by typing in your URL with /phpinfo.php at the end of the URL Example: www.yourwebsite.com/phpinfo.php

<?php
phpinfo();
?>

Are the Wordpess Plugins you are using main stream widgets or are they custom WordPress widgets? I assume your widgets are supposed to work with PHP5 by the way you phrased your question.

This could possibly solve the PHP5 problem so it is worth a try.

You should already have an .htaccess file for your WordPress website. If you don’t then automatically generate one for your WordPress website by going to your Settings panel in your WordPress Dashboard, then click on Permalinks and then click the Save Changes button. This should create an .htaccess file for your WordPress website and add it to the root of your domain.

Now download the .htaccess file via FTP and add this command to the very top of the new .htaccess file.

SetEnv PHP_VER 5

*** you may need to add a handler to your .htaccess fil as well – this just depends on your host so you will need to check with them or their FAQ/Help files.

AddHandler x-httpd-php5 .php
or maybe
AddType x-mapp-php5 .php

Also there could be other problems with your .htaccess file if you already had one that could be causing this problem.

The other thing I thought of – this is an obscure outside possibility, but worth checking – if you have a WordPress caching plugin installed you need to delete the cache and disable the plugin and then re-enable it.

Official WordPress Resource Page on Switching to PHP5