Install older version of php with yum

Hi to all,

I try to install on Centos with YUM a older version of php. When i use YUM LIST PHP i get the following results:

Available Packages
php.i386                                     5.1.6-23.2.el5_3                                      base
php.i386                                     5.1.6-24.el5_4.5                                      updates
php.i386                                     5.2.12-1.el5.art                                      atomic
php.i386                                     5.2.12-2.el5.art                                      atomic
php.i386                                     5.2.12-3.el5.art                                      atomic
php.i386                                     5.2.13-1.el5.art                                      atomic

which is great. now I want to install version 5.1.6 and tried all kind of things but he always give me the awnser that the package is not avalible or he wants to install 5.2.12

I tried: yum --allow-downgrade install php.i386-5.1.6-23.2.el5_3

I get the anwser:
Setting up Install Process
No package php.i386-5.1.6-23.2.el5_3 available.
Nothing to do

I am out of ideas here...
How can i choose which package i want to install with yum?

thanks for help,
Oliver