How to run or call for a composer with Plesk's precompiled PHP?
Composer run on base php of the OS. That's why it could lack necessary module, like mcrypt, mbstring.
Plesk has already compiled PHP with all necessary modules. In order to run composer with Plesk's precompiled PHP, use the following command:
/opt/plesk/php/5.6/bin/php /usr/lib64/plesk-9.0/composer.phar --help