Plesk (both Windows and Linux) uses its own internal PHP which does not depend on PHP installation used by hosted websites.
Installing PHP or Zend modules will not affect Plesk as long as panel's php.ini (
/usr/local/psa/admin/conf/php.ini on Linux and
%plesk_dir%\admin\php.ini on Windows) remains untouched by installation. Make a copy of this file before installing third-party PHP or Zend modules.