WordPress: add_meta_box error

Solution:

add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );
function myplugin_add_meta_box() {
    add_meta_box('myplugin_sectionid', 'Testing', 'myplugin_meta_box_callback', 'page' );
}
function myplugin_meta_box_callback ( $post ) {
    wp_nonce_field( 'myplugin_save_meta_box_data', 'myplugin_meta_box_nonce' );
    $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" />';
};

Declare myplugin_meta_box_callback as a function, not as a variable.