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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.
Using a natural language search will almost certainly be a better solution and PHP may not be the best tool for this task. Figure out how you are going to get the text out of the PDF and where you are going to put it. Look at things like sphinx and full text search in boolean mode for doing the keyword matching.
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.