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WordPress-Plugin-Boilerplate
[WordPress] A foundation for WordPress Plugin Development that aims to provide a clear and consistent guide for building your plugins.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
https://pippinsplugins.com/learn/. I learned quite a bit from Pippin. I use the WordPress Plugin Boilerplate when I create custom plugins (https://wppb.io/). If you go that route, you'll end up learning OOP PHP in the process.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.