Create a .php file with the following, put it on your web host, and access it from your browser. It should include all the version and configuration info you need to see if your hosting account meets those requirements:
<?php
phpinfo();
(If that shows that you are running PHP 4.x, try changing the file name suffix to ".php5" and see if that makes a difference, as some hosts still support both PHP4 and PHP5 that way.)