The given categories are checked against the post’s categories’ term_ids, names and slugs. Categories given as integers will only be checked against the post’s categories’ term_ids.
Prior to v2.5 of WordPress, category names were not supported. Prior to v2.7, category slugs were not supported. Prior to v2.7, only one category could be compared: in_category( $single_category ). Prior to v2.7, this function could only be used in the WordPress Loop. As of 2.7, the function can be used anywhere if it is provided a post ID or post object.
For more information on this and similar theme functions, check out the Conditional Tags article in the Theme Developer Handbook.
(int|string|array) (Required) Category ID, name or slug, or array of said.
(int|object) (Optional) Post to check instead of the current post.
Default value: null
(bool) True if the current post is in any of the given categories.