Hello Bogdan,
Do you use Plesk 12.5 and our new integration with composer applications in panel UI?
If not, I strongly recommend to upgrade Plesk and try it.
If you use composer from command line, you should add correct path to php binary in PATH env.
For example, my system php is 5.5:
$ php -v
PHP 5.5.9-1ubuntu4.11 (cli) (built: Jul 2 2015 15:23:08)
$ which php
/usr/bin/php
But I want to use php 5.6 installed by Plesk:
$ /opt/plesk/php/5.6/bin/php -v
PHP 5.6.13 (cli) (built: Sep 8 2015 11:53:39)
$ export PATH=/opt/plesk/php/5.6/bin:$PATH
$ php -v
PHP 5.6.13 (cli) (built: Sep 8 2015 11:53:39)
$ which php
/opt/plesk/php/5.6/bin/php