I tried to upgrade my server using:

yum upgrade

I received this error:

php-5.3.13-5.el5.art.x86_64 from atomic has depsolving problems
--> php conflicts with php-zend-optimizer
Error: php conflicts with php-zend-optimizer
You could try using --skip-broken to work around the problem
You could try running: package-cleanup --problems
package-cleanup --dupes
rpm -Va --nofiles --nodigest

So, it is conflict between zend optimizer and newest version.

I used suggested command:

yum upgrade --skip-broken

At the end of list there were this information:

Skipped (dependency problems):
libedit x86_64 20090923-3.0_1.el5.rf rpmforge 272 k
php x86_64 5.3.13-5.el5.art atomic 2.8 M
php-devel x86_64 5.3.13-5.el5.art atomic 1.3 M

At the end of Transaction test I receive this error:

Transaction Check Error:
file /usr/bin/phpize from install of php-cli-5.3.13-5.el5.art.x86_64 conflicts with file from package php-devel-5.2.17-1.el5.art.x86_64
file /usr/share/man/man1/phpize.1.gz from install of php-cli-5.3.13-5.el5.art.x86_64 conflicts with file from package php-devel-5.2.17-1.el5.art.x86_64

Zend information is this:

Name : php-zend-optimizer
Arch : x86_64
Epoch : 1
Version : 3.3.9
Release : 2.el5.art
Size : 5.8 M
Repo : installed
Summary : PHP Zend Optimizer
URL : http://www.zend.com/en/products/guard/downloads
License : Commercial
Description: PHP Zend Optimizer

I have Plesk, and I thought that upgrading Plesk would solve this problem, but it didn't. Plesk now is latest version. I have PHP 5.2.17, as you can see. On my other server I use for testing purposes upgrade went fine, but on production server there's this problem.

How can I solve this problem without reinstalling server?

Last I checked, php zend optimizer does not support php 5.3.x. You'll need to remove it

Yes, Zend Guard Loader is its replacement, but it's not backwards compatible. If you want to run encoded applications you'll need to get versions that can be decoded by Zend Guard Loader instead.

Thank you, I've removed zend optimizer and now everything's ok.

I have clients on my server who use applications that can only run under php zend optimizer.
What is the most recent version of php that I can install?

PHP 5.2.x. I wouldn't recommend running PHP 5.2.x in production though, since that's no longer supported. If you're running RHEL/CentOS 5 you could go with their PHP 5.1.x packages which are still supported by the distribution.

With PHP 5.3 they introduced Zend Guard Loader, which also supports running encoded apps, but is not compatible with apps which were encoded for use with Zend Optimizer. I'd recommend checking whether a version of the application that is compatible with Zend Guard Loader is available.

This is what I'm stuck with at the moment.