The require command adds new packages to the composer.json file. A composer.json file will be created if one doesn’t exist. After adding/changing the requirements, the modified requirements will be installed or updated.
--update-with-dependencies (-w): Also update dependencies of the newly required packages, except those that are root requirements.
--update-with-all-dependencies (-W): Also update dependencies of the newly required packages, including those that are root requirements.
Circumvent php memory limit
php -d memory_limit=-1 composer.phar install
Install Composer phar
curl -sS https://getcomposer.org/installer | php
Update a single package
composer update doctrine/doctrine-fixtures-bundle