VuFind has been bundled with an OAI-PMH harvest tool for a long time. It occurred to me that this tool might be of more general interest, so I have broken it out into a stand-alone project here:
This provides a simple command-line interface for downloading records from an OAI-PMH repository into a directory, including support for incremental updates. All of the details are documented in the README on GitHub.
The project can also be pulled in as a Composer dependency so that harvests can be controlled by other applications; this is how VuFind will continue to utilize the functionality in future releases.
If anyone has questions about this project, please let me know!