Index: functions.php
===================================================================
--- functions.php	(revision 6446)
+++ functions.php	(working copy)
@@ -485,6 +485,50 @@
 		fclose( $fp );
 }
 
+/**
+ * wp_mark_deprecated() - Marks a function as deprecated and informs when it has been used.
+ *
+ * There is a hook that will be called that can be used to get the backtrace up to what file
+ * and function called the deprecated function. That behavior is not defined here, because of
+ * preformance.
+ *
+ * The current behavior is to trigger an user error if WP_DEBUG is defined and is true. In later
+ * versions, this behavior could be changed to a more reasonable and less drastic approach of
+ * letting the user know that deprecated functions were used. However, normal usage will NOT
+ * trigger these errors, so only those that purposefully enable WP_DEBUG will see them.
+ *
+ * This function could be used in every place, including hooks that allows for calling the function.
+ * The common use will be for functions but can be for hooks, parameters, etc that is deprecated.
+ *
+ * @link http://trac.wordpress.org/ticket/4361 History of this function
+ *
+ * @package WordPress
+ * @package Debug
+ * @since 2.4
+ *
+ * @uses do_action() Calls 'deprecated_function_run' and passes the function name and what to use instead.
+ * @uses apply_filters() Calls 'deprecated_function_trigger_error' and expects boolean value of true to do trigger or false to not trigger error.
+ *
+ * @param string $used The function that was called
+ * @param string $seeinstead The function that should have been called
+ */
+function wp_mark_deprecated($used, $seeinstead) {
+	// Allow plugin to overwrite the error => forward thinking
+	$outputError = true;
+
+	// We can't know if another function will trigger this function before plugins.php is included.
+	// Test anyway. Small performance lost is okay since this function should not be called at all.
+	if( function_exists('do_action') ) {
+		do_action('deprecated_function_run', $function, $seeinstead);
+
+		// Allow plugin to filter the output error trigger
+		$outputError = apply_filters( 'deprecated_function_trigger_error', $outputError );
+	}
+
+	if( $outputError && defined('WP_DEBUG') && true === WP_DEBUG )
+		trigger_error( printf( __("%s is <strong>deprecated</strong>! Use %s instead."), $function, $seeinstead );
+}
+
 function do_enclose( $content, $post_ID ) {
 	global $wpdb;
 	include_once( ABSPATH . WPINC . '/class-IXR.php' );