Create single.php for specific category by category id

You can use this function to add category specific single template pages on your website. This goes in functions.php

You can define as many single templates as you want.

function wpse_category_single_template( $single_template ) {
    global $post;
    $all_cats = get_the_category();

    if ( $all_cats[0]->cat_ID == '1' ) {
        if ( file_exists(get_template_directory() . "/single-cat1.php") ) return get_template_directory() . "/single-cat1.php";
    } elseif ( $all_cats[0]->cat_ID == '2' ) {
        if ( file_exists(get_template_directory() . "/single-cat2.php") ) return get_template_directory() . "/single-cat2.php";
    }
    return $single_template;
}
add_filter( 'single_template', 'wpse_category_single_template' );

Here I used single-cat1.php for category id 1 and single-cat2.php for category id 2. You can name these single templates as you feel right.

This function also uses default fallback to single.php if there is no single-cat1.php or single-cat2.php exists.

EDIT 1

I have been using above code on 2 of my websites and it’s working fine on latest version of WordPress.

Paste this in your functions.php

function show_template() {
    global $template;
    print_r($template);
}
add_action( 'wp_head', 'show_template' );

This will print the template file used on each page/post. Now access your website and see if correct template file is being used or not? If it’s still showing single.php then there is something wrong with your code.

EDIT 2

Here is your code for that.

function wpse_category_single_template( $single_template ) {
    global $post;
    $all_cats = get_the_category();

    if ( in_category(6) ) {
        if ( file_exists(get_template_directory() . "/page-custom.php") ) {
          return get_template_directory() . "/page-custom.php";
        } else {
          return get_template_directory() . "/page.php";
        }
    }
    return $single_template;
}
add_filter( 'single_template', 'wpse_category_single_template' );