add element with add_action to posts from a specific category ID

I just try to add some content at the bottom of each posts from a specific category but the content is added in all posts. I use a hook in GeneratePress theme. I don’t understand why it is not working. I’m not a pro with PHP to be honest (maybe it’s obvious 😅). Thanks

function bottom_post_message() {
    $query = new WP_Query( array( 'cat' => '3,5' ) );
    if(is_single() && is_category($query)) {
        echo '<p>Hello World!</p>';
    }
}
add_action( 'generate_after_entry_content', 'bottom_post_message' );

Answers:

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Method 1

I just try to add some content at the bottom of each posts from a
specific category but the content is added in all posts.

Are you sure your “Hello World!” text is added in all posts? Because is_single() && is_category() would never be true because a query can’t be for a single post and a category archive at the same time. 🤔

So if you want to check if the current (or a specific) post is within a specific category, you would instead use in_category() like so:

function bottom_post_message() {
    // Check if the current request/URL is for a single post, and that the post is
    // in any of the categories passed to in_category().
    if ( is_single() && in_category( array( 3, 5 ) ) ) {
        // your code
    }
}

And note that you incorrectly used is_category() — you should pass the category IDs and not an instance of WP_Query.

See the documentation for is_single() and is_category() for more details about the functions (what they do, correct syntax, etc.).

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