Plesk Install PHP
To install differnt version of PHP in plesk server, run
plesk installer --select-release-current --show-components
This will list available PHP versions to install.
To install PHP 5.3, run
plesk installer --select-release-current --install-component php5.3
To install PHP 5.6, run
plesk installer --select-release-current --install-component php5.6
See Plesk