First, as you are apparently using OpenVZ and not true CentOS beware of any advice from here.

Other Voices
When CentOS Is Not CentOS

You were trying to install php, possibly of a different version, when php is already installed. Did you try "yum update"?

If more help is needed then please provide more information about your system by running "./getinfo.sh" and showing us the output file.

yum install php
Loaded plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
* base: centos.mirror.lstn.net
* extras: dist1.800hosting.com
* openvz-kernel-rhel5: mirror.fdcservers.net
* openvz-utils: mirror.fdcservers.net
* updates: centosv.centos.org
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package php.x86_64 0:5.1.6-32.el5 set to be updated
--> Processing Dependency: php-common = 5.1.6-32.el5 for package: php
--> Processing Dependency: php-cli = 5.1.6-32.el5 for package: php
--> Finished Dependency Resolution
php-5.1.6-32.el5.x86_64 from base has depsolving problems
--> Missing Dependency: php-common = 5.1.6-32.el5 is needed by package php-5.1.6-32.el5.x86_64 (base)
php-5.1.6-32.el5.x86_64 from base has depsolving problems
--> Missing Dependency: php-cli = 5.1.6-32.el5 is needed by package php-5.1.6-32.el5.x86_64 (base)
Error: Missing Dependency: php-cli = 5.1.6-32.el5 is needed by package php-5.1.6-32.el5.x86_64 (base)
Error: Missing Dependency: php-common = 5.1.6-32.el5 is needed by package php-5.1.6-32.el5.x86_64 (base)
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
The program package-cleanup is found in the yum-utils package.
[root@sever1 ~]# yum install php-common
Loaded plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
* base: centos.mirror.lstn.net
* extras: dist1.800hosting.com
* openvz-kernel-rhel5: mirror.fdcservers.net
* openvz-utils: mirror.fdcservers.net
* updates: centosv.centos.org
Setting up Install Process
Package matching php-common-5.1.6-32.el5.x86_64 already installed. Checking for update.
Nothing to do

I've tried yum clean all and it doesn't seem to work :/

Thanks