Here, this line is semantically identical to `if (isset($value['url'])) {` which is also much faster.
To summarise:
* array_key_exists: returns true iff array key exists, even when value is null
* isset: returns true iff array key exists AND value is not null
* empty: returns true iff array key exists AND value is not blank (null, empty string, 0, false...).
None of them raise an exception (or error, warning), when key does not exist they simply return false in this case.

The fastest is `isset`, followed by `empty`, and `array_key_exists` is much slower.