I'd recommend you use the distro supplied php packages. php-soap is a valid package. Seems like you installed php 5.2.x from another repository. Do not use packages from an older version of PHP on a newer one. This will have compatibility issues.
Do a,
# yum clean all
# yum search php-soap
# yum install php-soap
If the above doesn't work, there are multiple alternatives,
Check if the repository where you downloaded php-5.2, has a php-soap package
Upgrade to CentOS 6.3 (if this is an option) for later version of php from the distro which is php-5.3.3
Stick with CentOS 5.6 and use distro supplied php 5.1 packages
There are other 3rd party repositories offering php53 and php54 packages that you may want to try that might have php-soap
Compile php by yourself including the php-soap package from source