Description:
------------
Test-script below is expected to output the following (according to documentation at https://www.php.net/manual/de/function.array-merge.php) as the keys of the input-arrays are strings

Array
(
  [54984941951919] => aa
  [549849419519193] => bb
)

I get this result only on my old Raspberry running PHP 7.3.19-1~deb10u1.
On other servers (Windows witth 7.4.14; Docker with 7.4.14, 8.0.1, 7.3.19 and 7.3.26; even on an Ubuntu 20.04 with 7.3.19-1~deb10u1) the result is

Array
(
    [0] => aa
    [1] => bb
)

with is for arrays with numeric keys.
Yes, the keys from Test-script can be converted to integer but was defined as string key.
At least I expect a consistent behaviour on all PHP-versions and a hint within the documentation about string keys beeing convertable to integers.

I've found some older bugs (#25470, #18075, #13478, #54140) about this behaviour with #54140 stating that in the manual I should read "Strings containing valid integers will be cast to the integer type." but right now it is not in the mentioned documentation anymore.

Test script:
---------------
$arra = array('54984941951919'=>'aa');
$arrb = array('549849419519193'=>'bb');
echo '<pre>';
print_r( array_merge($arra, $arrb) );

Expected result:
----------------
Array
(
  [54984941951919] => aa
  [549849419519193] => bb
)

Actual result:
--------------
Array
(
    [0] => aa
    [1] => bb
)