Change the template used for singleposts based on their category, switching to a specific custom FSE (Full Site Editing) template when applicable. WordPress

functions.php

<?php
/**
 * Change the template used for single posts based on their category,
 * switching to a specific custom FSE (Full Site Editing) template when applicable.
 * 
 * Uses the `single_template_hierarchy` filter to insert the custom template 
 * at the top of the hierarchy when the post belongs to the target category.
 * 
 * This method works:
 * - On the front end (normal post display)
 * - In the Site Editor (edit/preview mode)
 * 
 * WordPress automatically looks for FSE templates in the hierarchy via `resolve_block_template()`.
 * 
 * ---
 * CONTEXT & EXAMPLE
 * ---
 * This snippet demonstrates how to dynamically assign a Full Site Editing template 
 * for single posts belonging to a specific category.
 * 
 * Example:
 * - Target category: “Actualités”
 * - Category ID: 3
 * - Category slug: `actualites`
 * - Custom FSE template: `Single actualites`
 * - Custom FSE template slug: `wp-custom-template-single-actualites`
 * 
 * When a post belongs to this category, WordPress will use the 
 * `wp-custom-template-single-actualites` block template 
 * (defined in the Site Editor or in the theme’s `wp_template` post type).
 * 
 * For all other posts, the default single template hierarchy is preserved.
 * 
 * ---
 * @param array $templates The current template hierarchy.
 * @return array The modified hierarchy with the category-specific custom template added first.
 * 
 * @author Thierry Pigot - www.wearewp.pro
 * @license MIT
 */
function twentytwentyfive_child_change_single_template( $templates ) {
	// Make sure this is indeed a single template hierarchy.
	// The first element should be 'single' or start with 'single-'.
	if ( empty( $templates ) || ( ! str_starts_with( $templates[0], 'single' ) && 'singular' !== $templates[0] ) ) {
		return $templates;
	}
	
	// Retrieve the current post (works both on the front end and inside the Site Editor via REST API).
	$current_post = null;
	
	// First, try to get it with get_queried_object() (front end).
	if ( is_singular( 'post' ) ) {
		$current_post = get_queried_object();
	}
	
	// If we're in the Site Editor, try to get the post from the REST request.
	if ( ! $current_post && isset( $_GET['postId'] ) && is_numeric( $_GET['postId'] ) ) {
		$current_post = get_post( (int) $_GET['postId'] );
	}
	
	// If we still don't have a post, try to infer it from the slug.
	if ( ! $current_post && ! empty( $templates[0] ) && str_starts_with( $templates[0], 'single-post-' ) ) {
		// The slug is like 'single-post-{slug}', so let's try to get the post from that.
		$post_slug = str_replace( 'single-post-', '', $templates[0] );
		$current_post = get_page_by_path( $post_slug, OBJECT, 'post' );
	}
	
	// If no post was found, do nothing.
	if ( ! $current_post || ! isset( $current_post->ID ) || 'post' !== $current_post->post_type ) {
		return $templates;
	}
	
	// Check if the post belongs to the "Actualités" category (ID: 3).
	// You can use either the ID or the slug.
	if ( ! has_category( 3, $current_post->ID ) && ! has_category( 'actualites', $current_post->ID ) ) {
		return $templates;
	}
	
	// Slug of the custom FSE template (without the 'wp_template/' prefix).
	// Example: 'wp-custom-template-single-actualites' as found in the editor URL.
	$custom_template_slug = 'wp-custom-template-single-actualites';
	
	// Check if the custom template exists before adding it.
	$theme_slug = get_stylesheet();
	$template_id = $theme_slug . '//' . $custom_template_slug;
	$custom_template = get_block_template( $template_id, 'wp_template' );
	
	// If the template exists, prepend it to the hierarchy.
	if ( $custom_template && ! empty( $custom_template->content ) ) {
		array_unshift( $templates, $custom_template_slug );
	}
	
	return $templates;
}
add_filter( 'single_template_hierarchy', 'twentytwentyfive_child_change_single_template', 10, 1 );