diff --git a/SeedDMS_Core/Core/inc.ClassDMS.php b/SeedDMS_Core/Core/inc.ClassDMS.php
index 06d7a6d13..45e93971f 100644
--- a/SeedDMS_Core/Core/inc.ClassDMS.php
+++ b/SeedDMS_Core/Core/inc.ClassDMS.php
@@ -271,6 +271,7 @@ class SeedDMS_Core_DMS {
 		$this->maxDirID = 0; //31998;
 		$this->enableConverting = false;
 		$this->convertFileTypes = array();
+		$this->callbacks = array();
 		$this->version = '@package_version@';
 		if($this->version[0] == '@')
 			$this->version = '4.3.26';
@@ -1291,8 +1292,9 @@ class SeedDMS_Core_DMS {
 
 		/* Check if 'onPostAddUser' callback is set */
 		if(isset($this->_dms->callbacks['onPostAddUser'])) {
-			$callback = $this->_dms->callbacks['onPostUser'];
-			if(!call_user_func($callback[0], $callback[1], $user)) {
+			foreach($this->_dms->callbacks['onPostUser'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $user)) {
+				}
 			}
 		}
 
@@ -1391,8 +1393,9 @@ class SeedDMS_Core_DMS {
 
 		/* Check if 'onPostAddGroup' callback is set */
 		if(isset($this->_dms->callbacks['onPostAddGroup'])) {
-			$callback = $this->_dms->callbacks['onPostAddGroup'];
-			if(!call_user_func($callback[0], $callback[1], $group)) {
+			foreach($this->_dms->callbacks['onPostAddGroup'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $group)) {
+				}
 			}
 		}
 
@@ -1479,8 +1482,9 @@ class SeedDMS_Core_DMS {
 
 		/* Check if 'onPostAddKeywordCategory' callback is set */
 		if(isset($this->_dms->callbacks['onPostAddKeywordCategory'])) {
-			$callback = $this->_dms->callbacks['onPostAddKeywordCategory'];
-			if(!call_user_func($callback[0], $callback[1], $category)) {
+			foreach($this->_dms->callbacks['onPostAddKeywordCategory'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $category)) {
+				}
 			}
 		}
 
@@ -1554,8 +1558,9 @@ class SeedDMS_Core_DMS {
 
 		/* Check if 'onPostAddDocumentCategory' callback is set */
 		if(isset($this->_dms->callbacks['onPostAddDocumentCategory'])) {
-			$callback = $this->_dms->callbacks['onPostAddDocumentCategory'];
-			if(!call_user_func($callback[0], $callback[1], $category)) {
+			foreach($this->_dms->callbacks['onPostAddDocumentCategory'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $category)) {
+				}
 			}
 		}
 
@@ -2272,7 +2277,20 @@ class SeedDMS_Core_DMS {
 	 */
 	function setCallback($name, $func, $params=null) { /* {{{ */
 		if($name && $func)
-			$this->callbacks[$name] = array($func, $params);
+			$this->callbacks[$name] = array(array($func, $params));
+	} /* }}} */
+
+	/**
+	 * Add a callback function
+	 *
+	 * @param string $name internal name of callback
+	 * @param mixed $func function name as expected by {call_user_method}
+	 * @param mixed $params parameter passed as the first argument to the
+	 *        callback
+	 */
+	function addCallback($name, $func, $params=null) { /* {{{ */
+		if($name && $func)
+			$this->callbacks[$name][] = array($func, $params);
 	} /* }}} */
 
 	/**
diff --git a/SeedDMS_Core/Core/inc.ClassDocument.php b/SeedDMS_Core/Core/inc.ClassDocument.php
index bdf0e33cb..d37252c67 100644
--- a/SeedDMS_Core/Core/inc.ClassDocument.php
+++ b/SeedDMS_Core/Core/inc.ClassDocument.php
@@ -1787,9 +1787,10 @@ class SeedDMS_Core_Document extends SeedDMS_Core_Object { /* {{{ */
 
 		/* Check if 'onPreRemoveDocument' callback is set */
 		if(isset($this->_dms->callbacks['onPreRemoveDocument'])) {
-			$callback = $this->_dms->callbacks['onPreRemoveDocument'];
-			if(!call_user_func($callback[0], $callback[1], $this)) {
-				return false;
+			foreach($this->_dms->callbacks['onPreRemoveDocument'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $this)) {
+					return false;
+				}
 			}
 		}
 
@@ -1874,8 +1875,9 @@ class SeedDMS_Core_Document extends SeedDMS_Core_Object { /* {{{ */
 
 		/* Check if 'onPostRemoveDocument' callback is set */
 		if(isset($this->_dms->callbacks['onPostRemoveDocument'])) {
-			$callback = $this->_dms->callbacks['onPostRemoveDocument'];
-			if(!call_user_func($callback[0], $callback[1], $this->_id)) {
+			foreach($this->_dms->callbacks['onPostRemoveDocument'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $this->_id)) {
+				}
 			}
 		}
 
diff --git a/SeedDMS_Core/Core/inc.ClassFolder.php b/SeedDMS_Core/Core/inc.ClassFolder.php
index 7d9a004f1..1155f3706 100644
--- a/SeedDMS_Core/Core/inc.ClassFolder.php
+++ b/SeedDMS_Core/Core/inc.ClassFolder.php
@@ -484,8 +484,9 @@ class SeedDMS_Core_Folder extends SeedDMS_Core_Object {
 
 		/* Check if 'onPostAddSubFolder' callback is set */
 		if(isset($this->_dms->callbacks['onPostAddSubFolder'])) {
-			$callback = $this->_dms->callbacks['onPostAddSubFolder'];
-			if(!call_user_func($callback[0], $callback[1], $newFolder)) {
+			foreach($this->_dms->callbacks['onPostAddSubFolder'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $newFolder)) {
+				}
 			}
 		}
 
@@ -802,8 +803,9 @@ class SeedDMS_Core_Folder extends SeedDMS_Core_Object {
 
 		/* Check if 'onPostAddDocument' callback is set */
 		if(isset($this->_dms->callbacks['onPostAddDocument'])) {
-			$callback = $this->_dms->callbacks['onPostAddDocument'];
-			if(!call_user_func($callback[0], $callback[1], $document)) {
+			foreach($this->_dms->callbacks['onPostAddDocument'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $document)) {
+				}
 			}
 		}
 
@@ -822,9 +824,10 @@ class SeedDMS_Core_Folder extends SeedDMS_Core_Object {
 
 		/* Check if 'onPreRemoveFolder' callback is set */
 		if(isset($this->_dms->callbacks['onPreRemoveFolder'])) {
-			$callback = $this->_dms->callbacks['onPreRemoveFolder'];
-			if(!call_user_func($callback[0], $callback[1], $this)) {
-				return false;
+			foreach($this->_dms->callbacks['onPreRemoveFolder'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $this)) {
+					return false;
+				}
 			}
 		}
 
@@ -880,8 +883,9 @@ class SeedDMS_Core_Folder extends SeedDMS_Core_Object {
 
 		/* Check if 'onPostRemoveFolder' callback is set */
 		if(isset($this->_dms->callbacks['onPostRemoveFolder'])) {
-			$callback = $this->_dms->callbacks['onPostRemoveFolder'];
-			if(!call_user_func($callback[0], $callback[1], $this->_id)) {
+			foreach($this->_dms->callbacks['onPostRemoveFolder'] as $callback) {
+				if(!call_user_func($callback[0], $callback[1], $this->_id)) {
+				}
 			}
 		}