composer update returns requirements could not be resolved

Question

$ composer update 
Loading composer repositories with package information
Updating dependencies (including require-dev)           
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - jenssegers/mongodb v3.3.1 requires mongodb/mongodb ^1.0.0 -> satisfiable by mongodb/mongodb[1.2.0, 1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.1.0, 1.1.1, 1.1.2].
    - jenssegers/mongodb v3.3.0 requires mongodb/mongodb ^1.0.0 -> satisfiable by mongodb/mongodb[1.2.0, 1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.1.0, 1.1.1, 1.1.2].
    - jenssegers/mongodb v3.3.1 requires mongodb/mongodb ^1.0.0 -> satisfiable by mongodb/mongodb[1.2.0, 1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.1.0, 1.1.1, 1.1.2].
    - mongodb/mongodb 1.2.0 requires ext-mongodb ^1.3.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.1.2 requires ext-mongodb ^1.2.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.1.1 requires ext-mongodb ^1.2.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.1.0 requires ext-mongodb ^1.2.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.0.5 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.0.4 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.0.3 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.0.2 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.0.1 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.0.0 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
    - mongodb/mongodb 1.2.0 requires ext-mongodb ^1.3.0 -> the requested PHP extension mongodb is missing from your system.
    - Installation request for jenssegers/mongodb ^3.3 -> satisfiable by jenssegers/mongodb[v3.3.0, v3.3.1].

  To enable extensions, verify that they are enabled in your .ini files:
    - /usr/local/etc/php/7.1/php.ini
    - /usr/local/etc/php/7.1/conf.d/php-memory-limits.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Answer 1

The problem is, that the mongodb extension is not installed our outdated.

Check the documentation to install mongodb for your operating system: https://secure.php.net/manual/en/mongodb.setup.php

and make sure the extension is enabled in your php.ini by checking for:

extension=mongodb.so

If the system you are using is different from where the code will run and that is why you don't have the extension installed you can also choose to ignore the platform requirements.