Say that PHP's start-up engine always uses MaxPerformance, and that Update Matched User Agent is on by default

src/devicedetection/cloudtoonpremise.md

@@ -166,6 +166,8 @@ Download new data files from the [51Degrees Distributor](@ref Services_Distribut
 - **LowMemory** - Slowest but minimal memory usage
 - **Balanced** - Default middle ground
 
+In PHP, the engine built when PHP starts always uses `MaxPerformance`, so there is no profile to choose. See [Performance options](@ref DeviceDetection_Features_PerformanceOptions) for the reason.
+
 **Deployment Options**
 
 - **In-Process** - Fastest option, detection in your application

src/devicedetection/features/falsepositivecontrol.md

@@ -40,10 +40,10 @@ By default, `Difference` and `Drift` are set to 0 (i.e., no deviation from the e
 
 # Match metadata
 
-Regardless of the settings used, the result will return additional metadata that can be queried to find out details about the match.
+Whatever the `Difference` and `Drift` settings, the result will return additional metadata that can be queried to find out details about the match.
 
 `Difference` - Contains the difference value between the supplied evidence and the returned match.  
-`UserAgents` - Contains a list of the matching substrings from the User-Agent.  
+`UserAgents` - Contains a list of the matching substrings from the User-Agent. The list is filled in only when the Update Matched User Agent setting is on, which it is by default.  
 `Drift` - Contains the drift value between the supplied evidence and the returned match.  
 `MatchedNodes` - The number of 'nodes' in the @hash tree where a match was found. If this is zero then no match was found.  
 `Method` - The method used to find a match. 'EXACT' indicates an exact match. 'NONE' indicates no match. Any other value is the name of the technique used to find the result that best matches the supplied evidence.

src/devicedetection/features/performanceoptions.md

@@ -26,6 +26,8 @@ The table below explains the options, from fastest performance and highest memor
 |Balanced (Default)|Some collections are loaded into memory at startup and the rest are read through caches of a fixed size. Frequently used items stay in memory, so disk reads are relatively uncommon for a typical web server workload.|Use for general workloads without extreme memory or performance requirements.|
 |LowMemory|Data always streamed from disk on-demand. | Recommended when the lowest possible memory usage is more important than performance. |
 
+The default is `Balanced`, except for the engine PHP builds when it starts. That engine is built from the php.ini file and always uses `MaxPerformance`, and any other profile set in the php.ini file is refused. PHP usually serves requests from worker processes copied from the process that built the engine. With a profile that reads from the data file, those workers would share one open data file and its read position, and corrupt each other's reads.
+
 The precise values associated with each template can be seen in the source code on [GitHub](https://github.com/51Degrees/51degrees-cxx/blob/67503df045efb32e84eb59fe7e320772dd6475db/src/hash/hash.c#L177).
 
 We measured throughput, startup time, and memory for Device Detection on seven machines, with the data file held in memory and with `Balanced`. The results are in the [hardware benchmarks](@ref ProductSummaries_Benchmarks_Hardware).

src/ipintelligence/algorithm.md

@@ -32,4 +32,6 @@ The algorithm is implemented in C for use in applications including edge computi
 - **Simple** - high-level language APIs make deployment simple for developers of all experience levels.
 - **Automatic Update** - data can be updated automatically without requiring a process restart.
 
+`Balanced` is the default profile, except for the engine PHP builds when it starts. That engine is built from the php.ini file and always holds all the data in memory with `MaxPerformance`, because PHP usually serves requests from worker processes copied from the process that built the engine, and with a profile that reads from the data file those workers would corrupt each other's reads. An engine that PHP code builds later, from a settings array, may use any profile and defaults to `Balanced`.
+
 See the [C code](https://github.com/51Degrees/51degrees-cxx/tree/main/src/ipi/graph) to find out more about the implementation.