The parallel\Runtime API provides a great degree of control to the power PHP programmer, and those intimately familiar with writing applications that use
parallel concurrency.
The functional API provides less control in exchange for the ability to make decisions for the programmer:
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all executing runtimes are bootstrapped identically
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scheduling is determined by the API, not the programmer
parallel\run provides the guarantee that the task will begin to execute in parallel as soon as allowed by hardware and operating system constraints, without
needlessly creating runtimes. For most applications the functional API should be preferred.