1

(english is not my language so I'm sorry if I don't explain really good)

My custom plugin get an URL from custom metabox and add it into the header.php My problem is that I can't get $post_id in my wp_head function to insert the URL in the header (all works fine before this).

Add my code from my custom plugin:

add_action( 'add_meta_boxes', 'extra_metabox' );

function extra_metabox () {

add_meta_box( 'extra-metabox', 'Extra Meta Box', 'extra_create_mb', array('page','post'), 'normal', 'core' );
}

function extra_create_mb( $post ){

wp_nonce_field( 'wpe_extra_nonce_save', 'wpe_extra_nonce' );

$meta_alternate = get_post_meta( $post->ID, 'extra_meta_alternate', true );

$html=
    '<table width="100%">
        <tr>
            <td style="width: 20%" valign="top"><label for="extra_meta_alternate">'.__('Alternate', 'extra_custom_alternate_redirect').'</label></td>
            <td>
                <input type="url" id="extra_meta_alternate" name="extra_meta_alternate" pattern="https://.*" value="' . esc_attr( $meta_alternate ) . '" style="width: 100%;">
            </td>
        </tr>
    </table>';


function extra_save_mb( $post_id ) {

$nonce_name   = isset( $_POST['wpe_extra_nonce'] ) ? $_POST['wpe_extra_nonce'] : '';
$nonce_action = 'wpe_extra_nonce_save';

if ( ! isset( $nonce_name ) ) {
    return;
}

if ( ! wp_verify_nonce( $nonce_name, $nonce_action ) ) {
    return;
}

if ( wp_is_post_revision( $post_id ) ) {
    return;
}

update_post_meta( $post_id, 'extra_meta_alternate', sanitize_text_field($_POST['extra_meta_alternate'] ) );

 }
add_action( 'save_post', 'extra_save_mb' );


function extra_head(){
//global $post;
$meta_alternate = get_post_meta( $post->ID, 'extra_meta_alternate', true );
//$meta_alternate = get_post_meta( $post_id, 'extra_meta_alternate', true );
//$meta_alternate = get_post_meta( get_the_ID(), 'extra_meta_alternate', true );
if ( is_page() || is_single() ){

        echo '<link rel="alternate" media="only screen and (max-width:400px)" href="'.$meta_alternate.'" />';   
        }   
}
add_action ('wp_head', 'extra_head');

But in <head></head> only print:

<link rel="alternate" media="only screen and (max-width: 400px)" href="">

Also I try to add global $post inside extra_head() or get_the_ID()but didn't work.

I'm relatively new with this, so thanks for the help.

1

Use get_queried_object_id() to get the post ID of the current page/post when viewing a single page or post, regardless of whether or not you're in the loop, or what the global $post variable happens to be at that moment:

if ( is_page() || is_single() ) {
    $post_id        = get_queried_object_id();
    $meta_alternate = get_post_meta( $post_id, 'extra_meta_alternate', true );

    echo '<link rel="alternate" media="only screen and (max-width:400px)" href="' . $meta_alternate. '" />';   
}

Note that I moved the get_queried_object_id() and get_post_meta() to inside the if statement. This is because if you're not on a page or post, the queried object ID could be the ID of a category or tag. This makes sure you're only getting the ID if you're on a post or page.

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