Run the following command to check all available versions of PHP:
# plesk bin php_handler --list
Then, choose the binary (including path) from the column php-cli , for example:
Note: Use the full path to launch binary in the command-line interface.
Here is simple example oh how to install symfony4 using composer:
/opt/plesk/php/7.1/bin/php /usr/local/bin/composer.phar create-project symfony/skeleton my_project