If you've worked with memcached and have ever run into an error, you might get tossed a code, the value of the error's constant, rather than the actual constant name. Paul Reinheimer was seeing just that and decided to map out all of the constants in relation to their error codes for future reference.
I was running into a few errors, and getResultCode() was obligingly returning the value of the constant, rather than the constant itself. I had to look up what that value meant. For some reason I couldn't reflect the extension to get the constants and values thereof, so with some lame scraping you get the following.
He's included the list in both a normal text format and as a PHP array that can be cut and pasted directly into a library.