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<?php

function ccd_footer_widgets( $params ) {	 

  global $footer_widget_num;

  if( isset($params[0]['id'] ) && $params[0]['id'] == 'footer_sidebar') {
    $footer_widget_num++;
    $divider = 3;
    
    if( $footer_widget_num % $divider == 0) {
      $class = 'class="last '; 
      $params[0]['before_widget'] = str_replace( 'class="', $class, $params[0]['before_widget'] );
    }
  }
  return $params;
}
add_filter( 'dynamic_sidebar_params','ccd_footer_widgets' );

?>