function wpsnipp_add_subtitle_metabox() {
    add_meta_box(
        'wpsnipp-subtitle', // Box ID
        __( 'Post Subtitle' ), // Box Title
        'wpsnipp_render_subtitle_metabox', // Callback function
        'post', // Post-type
        'normal', // Context
        'high' // Priority
    );
}
add_action( 'add_meta_boxes', 'wpsnipp_add_subtitle_metabox' );

function wpsnipp_render_subtitle_metabox() {
    // Add an nonce field for security.. we'll verify it later when saving the data
    wp_nonce_field( 'wpsnipp_subtitle_metabox', 'wpsnipp_subtitle_metabox_nonce' );
    // Retrieve the existing value (if any) to display in the field.
    $subtitle = get_post_meta( get_the_ID(), 'wpsnipp_post_subtitle', true );
    // Add our input field
    ?>
    <p><input type="text" class="widefat" id="wpsnipp-subtitle" name="wpsnipp-subtitle" value="<?php echo esc_attr( $subtitle ); ?>" /></p>
    <?php
}


// $post_id is passed into the function for us
function wpsnipp_save_subtitle( $post_id ) {

    // We need to verify this is the 'save_post' we're looking for
    // If our nonce is not found, we'll bail.
    if ( ! isset( $_POST['wpsnipp_subtitle_metabox_nonce'] ) ) {
        return;
    }

    $nonce = $_POST['wpsnipp_subtitle_metabox_nonce'];

    // If the nonce is not valid, we'll bail
    if ( ! wp_verify_nonce( $nonce, 'wpsnipp_subtitle_metabox' ) ) {
        return;
    }

    // If this is an autosave, our form has not been submitted, so we'll bail
    if ( defined( 'DOING_AUTOSAVE' ) &amp;&amp; DOING_AUTOSAVE ) {
        return;
    }

    // If the current user doesn't have permission, we'll bail.
    if ( ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }

     // OK, its safe for us to save the data now. 
    // Sanitize user input.
    $subtitle = sanitize_text_field( $_POST['wpsnipp-subtitle'] );

    // Update the meta field in the database.
    update_post_meta( $post_id, 'wpsnipp_post_subtitle', $subtitle );
}
add_action( 'save_post', 'wpsnipp_save_subtitle' );