Composer: how can I install another dependency without updating old ones?

To install a new package and only that, you have two options:

1. Using the require command, just run:

   composer require new/package
   

   Composer will guess the best version constraint to use, install the package, and add it to composer.lock.

   You can also specify an explicit version constraint by running:

   composer require new/package ~2.5
   

–OR–

2. Using the update command, add the new package manually to composer.json, then run:

   composer update new/package
   

If Composer complains, stating “Your requirements could not be resolved to an installable set of packages.”, you can resolve this by passing the flag --with-dependencies. This will whitelist all dependencies of the package you are trying to install/update (but none of your other dependencies).

Regarding the question asker’s issues with Laravel and mcrypt: check that it’s properly enabled in your CLI php.ini. If php -m doesn’t list mcrypt then it’s missing.

Important: Don’t forget to specify new/package when using composer update! Omitting that argument will cause all dependencies, as well as composer.lock, to be updated.