I have a cPanel server which we have used the 'express transfer' feature on to transfer the accounts. However, now it just downloads the index.php file and doesn't render it. How do I go about troubleshooting this?
Scroll down in CPanel to the Software/Services section, and open the PHP Configuration tab. Make sure that the .php option is enabled and set to use either PHP4 or PHP5.
Your mileage may vary depending on the CPanel version, but it shouldn't differ significantly.