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<?php
// Do NOT include the opening php tag

// Adds a box to the main column on the Post and Page edit screens. Goes to Dynamik Custom - Functions.
// From: https://codex.wordpress.org/Function_Reference/add_meta_box

function myplugin_add_meta_box() {

    $screens = array( 'post', 'page' );

    foreach ( $screens as $screen ) {

        add_meta_box(
            'mymetabox',
            __( 'This is my Custom Meta box', 'myplugin_textdomain' ),
            'myplugin_meta_box_callback',
            $screen
        );
    }
}
add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );

/**
 * Prints the box content.
 *
 * @param WP_Post $post The object for the current post/page.
 */
function myplugin_meta_box_callback( $post ) {

    // Add a nonce field so we can check for it later.
    wp_nonce_field( 'myplugin_save_meta_box_data', 'myplugin_meta_box_nonce' );

    /*
     * Use get_post_meta() to retrieve an existing value
     * from the database and use the value for the form.
     */
    $value = get_post_meta( $post->ID, '_my_meta_value_key', true );

    echo '<label for="myplugin_new_field">';
    _e( 'Description for this field', 'myplugin_textdomain' );
    echo '</label> ';
    echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="' . esc_attr( $value ) . '" size="25" />';
}

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function myplugin_save_meta_box_data( $post_id ) {

    /*
     * We need to verify this came from our screen and with proper authorization,
     * because the save_post action can be triggered at other times.
     */

    // Check if our nonce is set.
    if ( ! isset( $_POST['myplugin_meta_box_nonce'] ) ) {
        return;
    }

    // Verify that the nonce is valid.
    if ( ! wp_verify_nonce( $_POST['myplugin_meta_box_nonce'], 'myplugin_save_meta_box_data' ) ) {
        return;
    }

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Check the user's permissions.
    if ( isset( $_POST['post_type'] ) && 'page' == $_POST['post_type'] ) {

        if ( ! current_user_can( 'edit_page', $post_id ) ) {
            return;
        }

    } else {

        if ( ! current_user_can( 'edit_post', $post_id ) ) {
            return;
        }
    }

    /* OK, it's safe for us to save the data now. */
    
    // Make sure that it is set.
    if ( ! isset( $_POST['myplugin_new_field'] ) ) {
        return;
    }

    // Sanitize user input.
    $my_data = sanitize_text_field( $_POST['myplugin_new_field'] );

    // Update the meta field in the database.
    update_post_meta( $post_id, '_my_meta_value_key', $my_data );
}
add_action( 'save_post', 'myplugin_save_meta_box_data' );

<?php
// Do NOT include the opening php tag

// Combine with conditionals maybe! http://cobaltapps.com/forum/forum/main-category/web-design-talk/74915-additional-meta-boxes http://themefoundation.com/wordpress-meta-boxes-guide/ http://wptheming.com/2010/08/custom-metabox-for-post-type/

<?php        
// Retrieves the stored value from the database - put in hook box and choose hook location    
$meta_value = get_post_meta( get_the_ID(), '_my_meta_value_key', true );        
// Checks and displays the retrieved value      
if( !empty( $meta_value ) ) {          
echo $meta_value;      
}    
?>