Composer is a tool for dependency management in PHP. It allows you to declare the libraries your project depends on and it will manage (install/update) them for you.
composer require arkecosystem/client
Fork the package.
Clone forked repository.
git clone https://github.com/<githubusername>/php-client
Next, move into the cloned directory.
Install the dependencies with composer.
Dependencies are now installed, you can now run the tests to see if everything is running as it should.