Per the [https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#naming-conventions Naming Conventions]:
> Don’t abbreviate variable names unnecessarily; let the code be unambiguous and self-documenting.
This commit includes renaming of the following variables:
- `$post_ID` to `$post_id` — “Use lowercase letters in variable, action/filter, and function names”.
- `$ext_type` to `$extension_type`.
- `$ext` to `$extension`.
- `$id3data` to `$id3_data` — “Separate words via underscores”.
- `$msg` to `$message`.
- `$sb` to `$sidebar`.
- `$alt` to `$alternate`.
- `$pid` to `$post_id`.
- `$mid` to `$meta_id`.
- `$menu_obj` to `$menu_object`.
- `$noparents` to `$no_parents` — “Separate words via underscores”.
- `$sup` to `$supplemental`.
- `$cat_id` to `$category_id`.
- `$cat_name` to `$category_name`.
- `$out` to `$output`.
- `$r` to `$edit_result`.
- `$t` to `$taxonomy`.
- `$u` to `$update_result`.
- `$r` to `$result`.
- `$response` to `$response_data`.
- `$x` to `$response`.
Follow-up to [53723], [55365].
Props costdev, mukesh27, SergeyBiryukov.
See #64226.
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