CentOS

We have PHP 5.5.4 installed on CentOS 6.4 64-bit.

Installing mcrypt is proving to be a major issue.

Getting the error
[quote]
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/extensions/no-debug-non-zts-20121212/mcrypt.so' - /lib64/libc.so.6: version `GLIBC_2.14' not found (required by /usr/lib/php/extensions/no-debug-non-zts-20121212/mcrypt.so)
[/quote]

Does this mean that mcrypt needs glibc 2.14 or higher?:-?

If so, that would mean CentOS 6.4 can't run PHP 5.5.4 with mcrypt, and that just seems far-fetched.:-o
(CentOS 6.4 comes with glibc 2.12 and everyone strongly recommends against upgrading it)

We support what we ship. For php that is php 5.3.3

There is no sane third party repo that I know of that is yet shipping php 5.5, you'll only find it in places like webtatic!

My understanding is that mcrypt and libmcrypt for PHP 5.5.4 and 5.3.3 are the same. Is that incorrect?

It looks like libmcrypt is the same as that's the library that provides the functionality behind the scenes. But php-mcrypt is a release specific interface between the installed php and libmcrypt so has to match your installed php. For the CentOS php 5.3.3, php-mcrypt is helpfully supplied by EPEL. You'd have to grab the SRPM from them and rebuild it against your non-standard php to make it work