How to Select a Single Post Template by Category Slug or ID in WordPress

How It Works

When you need to assign a dedicated single post template to a specific category, you can add a custom function to your theme's functions.php file. WordPress provides the single_template filter, which allows you to dynamically select a template file based on conditions.

Implementation Steps

1. Create Template Directory and Files

In your current theme's root directory, create a folder named single. Then, create the corresponding template files for the categories you want to customize.

File Naming Rules:

• Using category slug: single-{category-slug}.php
• Using category ID: single-{category-id}.php

For example, if your category slug is "themes" and its ID is 2, you can create:

• single/single-themes.php
• single/single-2.php

The system will first try to match the slug-based file. If it doesn't exist, it will try the ID-based file.

2. Add the Template Selection Function

Add the following code to your current theme's functions.php file:

// Select custom single post template by category slug or ID
function wptoo_single_template($single) {
    global $post;
    // Get all categories for the current post
    foreach((array)get_the_category() as $cat) {
        // 1. First, check for a template file named by category slug
        $template_by_slug = TEMPLATEPATH . '/single/single-' . $cat->slug . '.php';
        if(file_exists($template_by_slug)) {
            return $template_by_slug;
        }
        // 2. If not found, check for a template file named by category ID
        $template_by_id = TEMPLATEPATH . '/single/single-' . $cat->term_id . '.php';
        if(file_exists($template_by_id)) {
            return $template_by_id;
        }
    }
    // If no custom template is found, return the default $single template path
    return $single;
}
// Hook the function to the 'single_template' filter
add_filter('single_template', 'wptoo_single_template');

Function Explanation and Corrections

The original code had a typo (ingle-) and room for logical optimization. The corrected code above:

1. Fixes the typo: Changed '/single/ingle-' to '/single/single-'.
2. Optimizes logic: Uses separate if statements for slug and ID checks, making the flow clearer.
3. Adds a default return: Returns the original $single path if no custom template is found, ensuring WordPress falls back to the default single post template (e.g., single.php).
4. Removes redundant variable: The unused $wp_query variable has been removed.

Result

After completing these steps, when viewing any post belonging to the "themes" category (or the category with ID 2), WordPress will automatically use single/single-themes.php (or single/single-2.php) as its single post template, enabling category-level template customization.