How can I replace WP post/blog single page from plugin?

There are a few different ways to replace the default single post template of a WordPress post or blog page from a plugin. One way to do this is by using the template_redirect action hook and the locate_template() function.

Here’s an example of how you could use this method to replace the default single post template with a custom template from your plugin:

function my_plugin_single_post_template( $template ) {
    if ( is_single() ) {
        $new_template = locate_template( array( 'single-post-custom.php' ) );
        if ( !empty( $new_template ) ) {
            return $new_template;
        }
    }
    return $template;
}
add_filter( 'template_include', 'my_plugin_single_post_template' );

This code uses the template_redirect action hook to check if the current page is a single post. If it is, it uses the locate_template() function to check for the existence of a custom template file named single-post-custom.php in the current theme’s directory. If the custom template file is found, it is used to display the single post page. If the custom template file is not found, the default single post template is used.

Another way to replace the post template is by using the template_include filter. This filter allows you to change the template that WordPress uses to display a post.

function my_custom_single_template( $single ) {
    global $post;
    if ( $post->post_type == 'post' ) {
        $single = plugin_dir_path( __FILE__ ) . 'templates/single.php';
    }
    return $single;
}
add_filter( 'single_template', 'my_custom_single_template' );

This code uses the template_include filter to check if the current post type is post, if it is the plugin’s single.php file is used to display the post.

It is important to note that you should use these methods inside a plugin and not in the theme. It is also important to make sure that your plugin’s template file is compatible with the current version of WordPress, and any other active plugin you are using.