I found 7.3 relatively uninteresting release in terms of new stuff.
You didn’t mention what I think is the best new feature: hrtime(). The interesting aspect is not so much the nanosecond resolution, which is nice, but that it is guaranteed to be linear with time. microtime(), otoh, returns the OS wallclock time, which may be adjusted unpredictably by the OS, so it is unreliable to measure elapsed time.
Yii should use hrtime(), when available, in its profiling features.