Check version of php. Stick in the root of your site where your WordPress resides. Then check it by visiting this page in your browser: yourdomain.com/what-is-php-version.php
<?php
// prints e.g. 'Current PHP version: 4.1.1'
echo 'Current PHP version: ' . phpversion();
// prints e.g. '2.0' or nothing if the extension isn't enabled
echo phpversion('tidy');