Recently we had the task of transferring a Magento Installation over from a CPanel server to a Plesk server. The transfer would have been rather textbook if not for the failure on the onepage checkout after the transfer was complete.

Symptom:
After transferring to the new server, when placing an order the checkout page would always redirect to the cart page rather than complete the order. No error messages or exceptions displayed by Magento.

What Happened Next?
The logs, always the logs. Actually no, we wasted a little time checking nothing had been hard coded into the javascript files used by the one page checkout, as we had not created it originally. That was to no avail, so then we checked the logs and found the following in var/log/system.log:

[code]2011-05-30T21:55:20+00:00 ERR (3): Notice: Use of undefined constant MCRYPT_BLOWFISH - assumed 'MCRYPT_BLOWFISH' in /www/domain.com/httpdocs/lib/Varien/Crypt/Mcrypt.php on line 56
2011-05-30T21:55:20+00:00 ERR (3): Notice: Use of undefined constant MCRYPT_MODE_ECB - assumed 'MCRYPT_MODE_ECB' in /www/domain.com/httpdocs/lib/Varien/Crypt/Mcrypt.php on line 60[/code]

Instantly twigged that php5-mcrypt was not installed on the server. As Magento was not installed via the installer and transfered instead this was not spotted sooner.

Installed php5-mcrypt and the onepage checkout worked once again. Another successful transfer. Another happy client.

If you have the same issue, around a year ago we coincidently wrote a blog post on how to install php5-mycrypt on a linux server. It may be some use to you in this situation.