I have added a custom post page option but it's not working; the output is always empty.

I have this debug code in my loop (the code that retrieves the information in frontend):

$colorvalue = get_post_meta( get_the_ID(), 'slider_is_dark', 1 );

echo '<script>alert("' . $colorvalue . '");</script>';

Then I have this in my functions.php:

add_action( 'add_meta_boxes', 'slider_checkboxes' );
function slider_checkboxes() {

function slider_box_content( $post_id ) {
    wp_nonce_field( plugin_basename( __FILE__ ), 'slider_nonce' );

    echo '<input type="checkbox" name="slider_is_dark" value="1" /> Slider is dark';

add_action( 'save_post', 'slider_data' );
function slider_data() {
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )

    if ( !wp_verify_nonce( $_POST['slider_nonce'], plugin_basename( __FILE__ ) ) )

    if ( isset( $_POST['slider_is_dark'] ) )
        update_post_meta( $post_id, 'slider_is_dark', 1 );
        update_post_meta( $post_id, 'slider_is_dark', 0 );

The alert always returns empty. Is there a problem in my code or am I doing it wrong?


You need the $post_id passed in to the function you are calling from the hook. Try making your function declaration like this:

function slider_data( $post_id ) 
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