Checking if a post is in a specific category can help developers serve functionalities based on categories. For example, changing a post style or loading a specific file for it, when it's in a specific category. the WordPress in_category can check if a post has a category checked for it or not.
In this tutorial, we’ll try using the WordPress in_category function in a working WordPress environment.
in_category function syntax
This function gets two parameters, the first one contains the categories, and the second one is the post.
$category parameter can be an integer, string, or an array containing multiple integers and strings (int[] or string[]).
$post parameter is optional; it can be a post ID or a post object.
If you do not specify $post, WordPress will consider the current post.
The WordPress
in_categoryfunction runs thehas_categoryfunction under the hood.
Check if a post is in specified categories using the in_category function  
In this example, we’re going to check if a post with id 1 is in the uncategorized category or not.
Change the $category to your category name, and $postId to your desired post’s id (use null for current post).
$category = "uncategorized"; //change the name
$postId = 1; //set null for current post
if(in_category($category, $postId)){
     //post is in the category
}You can change the $category value to an array.
WordPress developers can use array("uncategorized") value for $category and get the same result from the above example.
written by Mehdi Nazari about in WordPress WordPress Functions WordPress Plugin Development WordPress Theme Development
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