Many webservers run both php4 and php5 together.

Rather than bore you with the whys and wherefores – here is a simple solution that works on cpanel based servers.

Adding this line to your .htaccess file (remember to always make a backup!) which should be in your /public_html/ folder gives you php5

AddHandler application/x-httpd-php5 .php .php3 .phtml .php5