Theming global variables - initializing in index.php but using in header.pho

Question

I need to initialize some variables in various templates (index, single, page) such as banner image of that page. But then use that value in the 'get_header()' template. A good example of this is the og_image that's usually in the header.php set of meta tags inside the head of the HTML. This image is the visual moniker for any given template page. So the best place to get this info is in the context of the main loop of that template. However the og_image tag itself is not inside that template (such as single.php) but inside the header.php.

Setting a variable to "global" inside single.php doesn't help because these are not just straight includes as in regular Php. They are somehow more wordpress specific.

The other option could be to do some black magic inside functions.php, but for something this straightforward, I would prefer not to overdo some function. Is there an easier way or best practice to share variables' values across headed and footer and sidebar?

Answer 1

Using Custom Fields:
You can use custom fields for each page, if you need something more specific (maybe a product where you need an specific image for each page), taking the og values for example:

1.- Go to the edit page and check the custom fields checkbox so it can be seen below the editor

2.- Enter a new custom field

it should look like this, the name is your $key

3.- Save or Update the page.

4.- Add this code to your functions.php

function opengraph_tags() {

            /* we set the URL */
            $og_url = get_permalink();
            /* we set the site name */
            $og_site_name = get_bloginfo();
            /* we set the image */
            /* we check for a featured image */
            global $post;
            $og_img_src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'medium');
            $key = 'description';
            /* MY CUSTOM VALUE I WANT TO PUT FROM THE CONTENT TO THE HEADER */
            $og_description = get_post_meta($post->ID, $key , true);

        ?>
        <meta property="og:description" content="<?php echo $og_description; ?>"/>
        <meta property="og:type" content="website" />
        <meta property="og:url" content="<?php echo $og_url ?>"/>
        <meta property="og:site_name" content="<?php echo $og_site_name ?>"/>
        <meta property="og:image" content="<?php echo $og_img_src[0]; ?>"/>
        <?php
    }

add_action('wp_head', 'opengraph_tags', 5);

you can see i am grabbing the custom field with $key which can be anything you want, and also you can grab all the custom values you want, provided that you created them, you can set default values there too with a simple if(empty($myValue)), if you inspect the header you will see something like this:

Now if you want to use other values in header.php or even footer.php just set the custom field and use this code to grab the value:

<?php 
global $post;
$key = 'description';
/* MY CUSTOM VALUE I WANT TO PUT FROM THE CONTENT TO THE HEADER */
$og_description = get_post_meta($post->ID, $key , true);
?>

Answer 2

There is no black magic it is incredibly easy.

function globalvariabletest() {
    global $testvar;
    $testvar = 'hello world';
}
add_action( 'after_theme_setup', 'globalvariabletest' );