In WordPress, the functions in_category() and is_category() are both used to check if the current post belongs to a certain category. However, they are used in different contexts and have slightly different behavior.
The in_category() function is typically used within the WordPress Loop to check if the current post belongs to a specific category. It accepts either the category ID, slug, or name as a parameter and returns true if the post is in that category, and false otherwise. This function can be used in various template files, such as single.php, category.php, or archive.php, to conditionally display specific content based on the category of the post being displayed.
On the other hand, the is_category() function is usually used in the sidebar or other non-loop templates to determine if the current page being viewed is a category archive page. It does not accept any parameters and returns true if the current page is a category archive, and false otherwise. This function can be used in the sidebar.php file or any other template file that is not part of the loop.
Now, to answer your question about why in_category() also works similarly to is_category() in the sidebar code, it is because when the sidebar is being displayed, the current post or page being viewed is not available in the Loop context. Therefore, calling in_category() without any parameters within the sidebar code will actually check if the current page being viewed is a category archive page. This is because the in_category() function without parameters behaves like is_category() and checks the global category state of the current page.
In summary, the in_category() function is primarily used within the loop to check if the current post belongs to a specific category, while the is_category() function is used outside the loop to check if the current page being viewed is a category archive page. However, in some contexts, such as the sidebar, calling in_category() without any parameters will also check if the current page is a category archive.