Add missing parameter description in wp_get_post_parent_id function DocBlock

[keesiemeijer]

The wp_get_post_parent_id function accepts a post ID or a post object to get the parent post id and it defaults to the global post object.

Because the parameter is called $post_ID It needs to be updated to $post

[keesiemeijer]

Add missing param description and rename $post_ID to $post

[keesiemeijer]

In patch 42797.patch​ the $post_ID parameter is renamed to $post and the description is added. I've added unit tests to see if the original code and new code return the same parent post ids.

[SergeyBiryukov]

In 42397:

Docs: Add missing parameter description in wp_get_post_parent_id() function DocBlock.

Rename $post_ID to $post for consistency with other functions.

Add unit tests for wp_get_post_parent_id().

Props keesiemeijer.
Fixes #42797.

[SergeyBiryukov]

In 42398:

Posts: Rename $ID to $post in get_post_mime_type() and get_post_status() for consistency with other functions.

See #42797.

[anonymized_8769252]

Reopening this because the unit tests are not of an acceptable quality:

1. Shared fixtures should be used whenever possible. The test suite is already slow, and we should keep it from getting slower unnecessarily.
2. The global $post is an instance of WP_Post, and not an integer (post id).

Two smaller improvements:

1. assertSame() should be preferred, because the checks are stricter.
2. assertInstanceOf() is a shorter way to determine the type of an object.

Also the global $post gets reset after every test, so no need to do it manually.

[SergeyBiryukov]

In 42635:

Tests: Improve tests for wp_get_post_parent_id() added in [42397].

Props frank-klein.
Fixes #42797.