I'm guessing this is not possible, as wp_head() is probably finished before the content execution, but here's a quick example of what I'm trying to do:
add_shortcode( 'header_content', function () {
add_action('wp_head',function () {
echo '<meta name="test" content="hello world" />';
} );
} );
My actual use case is to have body content add JSON-LD to the header.
Is there a simple and elegant solution to this without a plugin?
No, by the time your shortcode runs, the header has already been sent over the network, even the post title. It is too late to change things already sent, aka everything before it reached the_content.
There is 1 way to "do it" sort of, but it carries a very, very high cost, and won't be reliable. Start an output buffer at the beginning of every page, then process the page at the end. However, this will:
shutdown hook, if this is never reached then nothing will be shown on the page, the user will see a completely blank page with no markup at allwp_kses_post on save. Fixing this will mean opening up major security holesFundamentally, this is not the approach to take to solve these problems
Instead, if you want to add JSON-LD to the header, you should use PHP code to add it on the wp_head hook, or if possible in the footer or further down the page. You shouldn't rely on shortcodes for anything other than embedding things into post content that can't be put directly into post content, e.g. iframes or fancy javascript features like interactive graphs
To add additional tags (like <style>, <script>, <meta>, or etc) to the <head> when a certain shortcode is used in the post, we can use the WordPress has_shortcode function:
function add_meta_tags() {
global $post;
if ( has_shortcode( $post->post_content, 'YOUR-SHORTCODE' ) ) {
echo '<meta name="test" content="hello world" />';
}
}
add_action('wp_head', 'add_meta_tags');
function shortcode_handler() {
// some codes
}
add_shortcode('YOUR-SHORTCODE', 'shortcode_handler');
