An example Textpattern plugin with an updater

abc_plugin_update.php

<?php

/**
 * The plugin class.
 */

class abc_plugin
{
	/**
	 * The plugin version.
	 *
	 * @var string
	 */

	static public $version = '1.2';

	/**
	 * Constructor.
	 */

	public function __construct()
	{
		register_callback(array($this, 'install'), 'plugin_lifecycle.'.__CLASS__, 'installed');
		register_callback(array($this, 'uninstall'), 'plugin_lifecycle.'.__CLASS__, 'deleted');
	}

	/**
	 * Installer.	
	 */

	public function install()
	{
		$version = (string) get_pref(__CLASS__.'_version', '');

		if (!$version)
		{
			// base install
		}

		if (version_compare($version, '1.1', '<'))
		{
			// update to 1.1
		}

		if (version_compare($version, '1.3', '<'))
		{
			// update to 1.3
		}

		// Write version number to database.

		set_pref(__CLASS__.'_version', self::$version, __CLASS__, PREF_HIDDEN);
	}

	/**
	 * Uninstaller.
	 */

	public function uninstall()
	{
		safe_delete(
			'txp_prefs',
			"name LIKE '".str_replace('_', '\_', __CLASS__)."\_%'"
		);
	}
}

new abc_plugin();

?>