[Php73] Add ArrayKeysToArrayKeyFirstLastRector

[Soean]

Adds ArrayKeysToArrayKeyFirstLastRector, which replaces reading a single key off array_keys() with the dedicated PHP 7.3 function:

$firstKey = current(array_keys($items));
$alsoFirst = reset(array_keys($items));
$lastKey = end(array_keys($items));

$firstKey = array_key_first($items);
$alsoFirst = array_key_first($items);
$lastKey = array_key_last($items);

array_keys() materializes every key just to have all but one thrown away. Beyond the wasted allocation, the intent is easier to read once the dedicated function is used.

This complements the existing ArrayKeyFirstLastRector, which is STMTS_AWARE and only matches the two-statement reset($items); key($items); form, the nested expression is not covered by it, so the two rules do not overlap.

Skipped cases

• array_keys() with a search value, array_keys($items, 'value') returns only the matching keys, so its first entry is not the array's first key. Transforming this would be a behavior change.
• Named arguments, spread, and first-class callables on either call.
• key(array_keys($items)), which always evaluates to 0 rather than the first key.

Note on empty arrays

current(array_keys([])) returns false, while array_key_first([]) returns null. The rule transforms regardless, matching how ArrayFirstLastRector handles the equivalent array_values($array)[0] → array_first($array) case. Gating on a proven non-empty-array type would leave nearly nothing to fix. Happy to add such a guard if you would rather have it.

[samsonasik]

The downgrade rule seems needed first at

https://github.com/rectorphp/rector-downgrade-php/

If not in existing downgrade rule functionality about it.

[TomasVotruba]

Thanks for the feature 👍

We already have a downgrade rule that handles this.
https://github.com/rectorphp/rector-downgrade-php/blob/main/rules/DowngradePhp73/Rector/FuncCall/DowngradeArrayKeyFirstLastRector.php