How to get the post-ID in my plugin?

Question

I am trying to write a little plugin which allows me to add a different theme to each page. well.. still trying.. ;)

code: http://pastebin.com/dP1GH43E

When I put return 'mytesttheme'; at the end it works just perfect! when I add an ID ($page_theme = get_post_meta('66', [...]) it also works..

when I write echo $page_theme.', '; before the return, it outputs: , , , mytesttheme, mytesttheme, mytesttheme, mytesttheme,

The empty spaces are the problem I think.

in wp-backend it shows "mytesttheme" as post meta content in wp-core post meta content box.. so the saving works as it should..

Any ideas how to solve the problem? :S

Answer 1

There are several issues with your code. Let's start at the beginning...

Adding hooks

All your hooks will be added at the backend. Even those, which are outside the if((is_admin)) branch. Add your hooks explicit where they are should work:

if (is_admin()){
  // do this on backend (admin side)
    /* load plugin functions */
    add_action('add_meta_boxes', 'myplugin_add_meta_box');
    add_action('save_post', 'myplugin_save_meta_box');

} else {
  // if we are NOT on backend (admin side) we have to be on frontend, because there is only frontend and backend.
    /* load page theme */
    add_filter( 'option_template', 'my_plugin_get_page_theme');
    add_filter( 'template', 'my_plugin_get_page_theme');
    add_filter( 'option_stylesheet', 'my_plugin_get_page_theme');

}

In the loop or not?

get_the_ID() must be within the loop. Where are your hooks ('option_template', 'template' and 'option_stylesheet') are done? Before the query is done or after (means: inside the loop or outside) ? Two lines of code give us the answer:

global $post;
var_dump( $post );

If $post is empty (null), we areoutside the loop. We hooked to early. We need a hook after the query is done. The ´init´ hook is a ood one. Let's alter the code above a bit and get the plugin working:

if (is_admin()){

    /* load plugin functions */
    add_action('add_meta_boxes', 'myplugin_add_meta_box');
    add_action('save_post', 'myplugin_save_meta_box');

} else {

    add_action( 'init', 'my_template_hooks', 5, 0 );

}

    function my_template_hooks(){

        /* load page theme */
        add_filter( 'option_template', 'my_plugin_get_page_theme');
        add_filter( 'template', 'my_plugin_get_page_theme');
        add_filter( 'option_stylesheet', 'my_plugin_get_page_theme');

    }

Now our hooks are inside the loop and the plugin works fine.

Deprecated function

On line 28 of your plugin code you use $themes = get_themes();. get_themes() is deprecated and should be replaced with ´wp_get_themes()´.

Development

Please enable WP-debugging (set WP_DEBUG to TRUE in your wp-config.php). The debug messages will help you much.

You can find the complete modified code on Github

Answer 2

As far I know, to use post->ID outside of loop, wp_query should be called first. I mean something like that the code bellow which is already suggested by @Jan

global $wp_query; 
$postid = $wp_query->post->ID;

Optionally, get_post_id() can work for you, check codex for more.