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CommonMark PHP
Highly-extensible PHP Markdown parser which fully supports the CommonMark and GFM specs.
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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.
It gets quite granular: https://github.com/thephpleague/commonmark/blob/latest/src/Extension/CommonMark/CommonMarkCoreExtension.php
Have you looked at HTML Purifier?
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.