Fix type of `wp_cache_set_last_changed()`

[tillkruss]

The timestamp for queries currently is stored as 0.91192600 1743003204, not as a numeric value. This makes it hard to compare, plus it consumes unnecessary bytes in the key name.

Trac ticket: https://core.trac.wordpress.org/ticket/63194

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[peterwilsoncc]

There are a number of plugins using the setter and associated getter (which would also be affected by the type change) in the plugin repo. I expect there are a few unlisted plugins using it too, for example the WordPress.com VIP MU Plugins.

Changing the type would affect plugins running something like this:

$lc = explode( ' ', wp_cache_(set|get)_last_changed('group') );
$time = $lc[1] + $lc[0];

With this change these would start throwing a warning as $lc[1] isn't set. I expect that this is exactly the code you wish to avoid running.

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A concern I have is that the return type of wp_cache_get_last_changed('group') would become unpredictable after this change. For frequently invalidated caches, such as posts, it would quickly become a float. For infrequently invalidated caches, such as users, it would remain a string.

This could be resolved by flushing the global cache upon upgrade, but that would miss a lot of enterprise type sites that don't run the upgrade script as frequently as they could.

[tillkruss]

@peterwilsoncc I agree. I changed the type of the filter to string so at least it's accurate now, it was never int. Thanks for checking.

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A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60128 GitHub commit: https://github.com/WordPress/wordpress-develop/commit/07bf0f91117629fb382ad09b6de1d138baea15e0

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.