This is a follow-up to [52292] which introduced `is_string()` to check the given key is a string to be sanitized, else the key is set to an empty string.
`sanitize_key()` is clearly identified (in the documentation) to only work with ''string'' keys. However, it had a bug in it that allowed non-strings to pass through it:
* A non-scalar "key" would throw a PHP Warning (which was resolved in [52292].
* A non-string scalar "key" was handled by the PHP native `strtolower()` which converted it into a string.
While `is_string()` is valid, non-string scalar types passed as the key to be sanitized were being set to an empty string. Given that `strtolower()` handles these without error or deprecation as of PHP 8.1, `is_scalar()` protects the website from issues while retaining the past behavior of converting integer keys (for example) into a string.
Changes include:
* Using `is_scalar()` instead of `is_string()`
* Refactor for readability and less code
* More tests
Please note, this does not change the behavior of the function, nor redefine it to now accept non-string scalars.
References:
* https://developer.wordpress.org/reference/functions/sanitize_key/
* https://www.php.net/manual/en/function.strtolower.php
Follow-up [52292].
Props wppunk, hellofromTonya, costdev, jrf.
Fixes#54160.
git-svn-id: https://develop.svn.wordpress.org/trunk@52370 602fd350-edb4-49c9-b593-d223f7449a82
d3a851d0d1