Description:
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There are places on the web telling me that array_key_exists($k, $a) is slower than isset($a[$k]). https://startpage.com/do/search?query=php+performance+isset+vs+array_key_exists&cat=web&pl=chrome&language=english

Tests with PHP 5.5.9 confirm this, it seems that array_key_exists() uses almost 4x the time of isset().

On the other hand: Semantically, array_key_exists() is preferable over isset().
If you already know that $a is an array and not null, and that $k is a valid array key, then you don't want to have this tested again. You want TRUE or FALSE if the variables are set, and a warning (and FALSE, I guess) otherwise.

This allows the IDE to warn you if you test a variable that is possibly undefined, or possibly not an array. With isset(), the IDE will keep silent, which can lead to hidden bugs.

In theory, array_key_exists() should be faster than isset(), because there are fewer cases to consider.. but it is not, because one is a function and the other a "language construct".

Aside of that, there is of course the difference of how NULL values are handled.. In fact, in an ideal world, there would be two versions of array_key_exists(). Both with the performance of a language construct. But with a different behavior for NULL values.