The issue is when working with large arrays or being called frequently, array_key_exists is 2x to 10x slower than isset.
Interesting, thanks! Do you have a benchmark available somewhere?
It's a simple benchmark with many examples, but to quote one of them provided on the PHP manual:
Benchmark (100000 runs):
array_key_exists() : 205 ms
is_set() : 35ms
isset() || array_key_exists() : 48ms
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