How to install PHP SOAP module on Plesk server?
With Plesk 12.0 and Plesk 12.5 , you can install and use multiple PHP versions built by the Plesk team.
PHP packages from Plesk team have SOAP module included. More information about multiply PHP support you can find in article #213949705 .
In Plesk 12 PHP packages from the Plesk team are available on CentOS 6, CentOS 7, and Ubuntu 14 only.
In Plesk 12.5 PHP packages from the Plesk team are available on RHEL 6, 7, CentOS 6,7, Ubuntu 12,14 and Debian 7,8 operating systems.
To install SOAP module for PHP by OS vendor, or on server with Plesk 11.5 and earlier you should contact with system administrator.Also the following links can be useful:
Please note that the above URL's are provided just for your reference.