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framework-x
Framework X – the simple and fast micro framework for building reactive web applications that run anywhere.
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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.
As others have mentioned, in certain scenarios using a long-running application server (rather than mod_php/php-fpm) to serve requests can make sense. There are a number of extensions and libraries available that facilitate this with asynchronous code execution - Swoole has already been mentioned but you may also want to check out ReactPHP (not related to JS React, other than they stole the name) and Framework X
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.