Fix array_walk() call on PHP 8

[derrabus]

Running version 6.7 of this library on PHP 8 will result in a warning being triggered:

Argument #2 ($key) must be passed by reference, value given

The fix is fairly trivial because the affected variable is not used and can be removed.

This issue has already been reported as #1085 fixed on higher branches. However the project I'm working with at the moment is currently unable to upgrade to Elasticsearch 7. And as far as I can tell, this issue seems to be our only blocker for upgrading to PHP 8.

If the 6.7.x branch is closed for bugfixes, please close this PR.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[alexander-schranz]

The ones which did subscribe this PR. Elasticsearch did release a PHP 8 supported Version with 7.11: https://github.com/elastic/elasticsearch-php/releases/tag/v7.11.0 not sure if they will support PHP 8 with older version maybe @ezimuel can make a statement about it.

[ezimuel]

The original plan was to support PHP 8 only for elasticsearch-php 7.x (I just released it yesterday with 7.11.0). Considering that elasticsearch 6.x is still quite popular I can consder to add the support of PHP 8 al well.

@derrabus can you send the PR to 6.x? Thanks!

[derrabus]

Sure, no problem. I've changed the target branch and rebased the changes.

If you need more help building a PHP8-compatible 6.x release, let me know.

[derrabus]

The Travis failures are unrelated to my changes. I'm trying to fix them in #1105.

[derrabus]

I think we can close the PR because the problem has been addressed in b316710. I've switched our project to the 6.8.x branch of this library and it works like a charm!

Thank you very much, @ezimuel!