Custom Metabox

function tp_custom_meta_boxes() {
    add_meta_box('tp_cs_meta', 'TP Meta', 'tp_custom_post_meta', 'post', 'side', 'high');
}
add_action( 'add_meta_boxes', 'tp_custom_meta_boxes' );


function tp_custom_post_meta( $post ) {
    $photo_credit = get_post_meta( $post->ID, '__photo_credit', true );
    $photo_credit = isset($photo_credit) ? $photo_credit : '';

    ?>
        <p>
            <div class="row-content">
            <label for="featured-post">
                <input type="text" name="photo_credit" id="photo_credit" value="<?php echo esc_attr($photo_credit); ?>" class="widefat">
                <?php _e( 'Featured', 'themeplate' )?>
            </label>
        </div>
        </p>
    <?php
}


// Save metadata
function tp_save_post_meta( $post_id, $post ) {
    //use this for the multiple post type
    $edit_capability = get_post_type_object( $post->post_type );

    if( !current_user_can(), 'edit_post', $post_id ) {
        return;
    }
    $is_autosave = wp_is_post_autosave( $post_id );
    $is_revision = wp_is_post_revision( $post_id );

    if ( $is_autosave || $is_revision ) {
        return;
    }

    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    if ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {
        return;
    }

    $photo_credit = !empty( $_POST['photo_credit'] ) ? sanitize_text_field($_POST['photo_credit']) : '';
    if( array_key_exists('photo_credit', $_POST ) ) {
        update_post_meta( $post_id, '__photo_credit', $photo_credit );
    }
}
add_action( 'save_post', 'tp_save_post_meta', 10, 2 );