Sorry, last commit broke the code. I had to make a separate plugin function.

functions/imap_mailbox.php

@@ -51,7 +51,7 @@ function isSpecialMailbox( $box ) {
              ($save_as_draft && $box == $draft_folder ) );
 
     if ( !$ret ) {
-        $ret = do_hook( 'special_mailbox', $box );
+        $ret = do_hook_function( 'special_mailbox', $box );
     }
 
     return( $ret );

functions/plugin.php

@@ -28,7 +28,7 @@ function use_plugin ($name) {
 }
 
 /* This function executes a hook. */
-function do_hook ($name,$parm=NULL) {
+function do_hook ($name) {
     global $squirrelmail_plugin_hooks;
     $data = func_get_args();
     $ret = '';
@@ -38,7 +38,27 @@ function do_hook ($name,$parm=NULL) {
         foreach ($squirrelmail_plugin_hooks[$name] as $function) {
             /* Add something to set correct gettext domain for plugin. */
             if (function_exists($function)) {
-                $ret = $function($data,$parm);
+                $function($data);
+            }
+        }
+    }
+
+    /* Variable-length argument lists have a slight problem when */
+    /* passing values by reference. Pity. This is a workaround.  */
+    return $data;
+}
+
+/* This function executes a hook. */
+function do_hook_function($name,$parm=NULL) {
+    global $squirrelmail_plugin_hooks;
+    $ret = '';
+
+    if (isset($squirrelmail_plugin_hooks[$name])
+          && is_array($squirrelmail_plugin_hooks[$name])) {
+        foreach ($squirrelmail_plugin_hooks[$name] as $function) {
+            /* Add something to set correct gettext domain for plugin. */
+            if (function_exists($function)) {
+                $ret = $function($parm);
             }
         }
     }
@@ -48,6 +68,7 @@ function do_hook ($name,$parm=NULL) {
     return $ret;
 }
 
+
 /*************************************/
 /*** MAIN PLUGIN LOADING CODE HERE ***/
 /*************************************/

plugins/filters/setup.php

@@ -72,7 +72,7 @@
       $squirrelmail_plugin_hooks['special_mailbox']['filters'] = 'filters_special_mailbox';
    }
 
-   function filters_special_mailbox( $data, $mb ) {
+   function filters_special_mailbox( $mb ) {
 
        if ( $mb == getPref($data_dir, $username, 'filters_spam_folder', 'na' ) ) {
            return( TRUE );