How does running 'composer install' differ from 'composer update' when a composer.lock file exists?

The commands composer install and composer update are used for managing dependencies in PHP projects, but they work differently when a composer.lock file is present:

• composer install:

  • Installs dependencies strictly in the versions locked in composer.lock.
  • Does not update package versions.
  • Used for reproducible installations, for example, in production.

• composer update:

  • Updates dependencies to the latest versions that satisfy the constraints in composer.json.
  • Overwrites composer.lock with new versions.
  • Used when updating packages during development.

Thus, composer install ensures that all developers and servers use the same library versions, while composer update allows obtaining the latest available updates.