It appears that there is a package update problem. php-imap wants
libc-client2006, which provides libc-client.so.2006. If libc-client2006 is
present, yum update attempts to replace it with libc-client, which does NOT
provide libc-client.so.2006. I had not observed a problem with this prior to
php-imap 5.2.4-3, so it may conceivably be a problem with that package, but this
smells like a case where yum update is incorrectly analyzing package replacement
rules.

I have marked this "high" because it breaks updates in general and because
php-imap is widely used.

You ought to be able to repeat this by:

  rpm -e libc-client2006 php-imap
  yum install php-imap # pulls in libc-client2006
  yum update # tries to update to libc-client