$plugins_dir
       This is the directory (or directories) where Smarty will look for the
       plugins that it needs. Default is
       "plugins" under the
       SMARTY_DIR. If you
       supply a relative path, Smarty will first look under the
       SMARTY_DIR, then
       relative to the cwd (current working directory), then relative to the PHP
       include_path. If $plugins_dir is an array of directories, Smarty will
       search for your plugin in each plugin directory in the order they are
       given.
      
Technical Note: 
          For best performance, do not setup your $plugins_dir to have to use
          the PHP include path. Use an absolute pathname, or a path relative
          to SMARTY_DIR or the cwd.
          
Example 12-1. Appending a local plugin dir 
<?php
  $smarty->plugins_dir[] = 'includes/my_smarty_plugins';
  ?>
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  | 
Example 12-2. multiple $plugins_dir 
<?php
  $smarty->plugins_dir = array(                        'plugins', // the default under SMARTY_DIR                        '/path/to/shared/plugins',                        '../../includes/my/plugins'                        );
  ?>
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  |