title: PHP Strict Errors
body: <p>I get these PHP errors in error_log all the time and can't figure out how to solve it:</p>
  
  <p>Errors:</p>
  
  <pre><code>PHP Strict Standards:  Non-static method wf_gmap::check_wp_footer() should not be called statically in /home/USERNAME/public_html/WEBSITE/wp-content/plugins/5sec-google-maps/5sec-gmaps.php on line 25
  
  PHP Notice:  Use of undefined constant is_admin - assumed 'is_admin' in /home/USERNAME/public_html/WEBSITE/wp-content/plugins/5sec-google-maps/5sec-gmaps.php on line 21
  
  PHP Strict Standards:  call_user_func_array() expects parameter 1 to be a valid callback, non-static method wf_gmap::init() should not be called statically in /home/USERNAME/public_html/WEBSITE/wp-includes/plugin.php on line 429
  </code></pre>
  
  <p>and 5sec-gmaps.php file's lines from 17 to 44 is this:</p>
  
  <pre><code>class wf_gmap {
    static $js_functions = '';
  
  public function init() {
  if(is_admin) {
    add_filter('plugin_row_meta', array(__CLASS__, 'plugin_meta_links'), 10, 2);
  }
  // check wp_footer()
  self::check_wp_footer();
  
  // add shortcode
  global $shortcode_tags;
  if (isset($shortcode_tags[GMAP_SHORTCODE])) {
    add_action('admin_footer', array(__CLASS__, 'warning'));
  } else {
    add_shortcode(GMAP_SHORTCODE, array(__CLASS__, 'shortcode'));
  }
  
  // add JS include files
  add_action('wp_footer', array(__CLASS__, 'footer'), 2);
  
  // add shortcode support in sidebar text widget
  if (has_filter('widget_text', 'do_shortcode') === false) {
    add_filter('widget_text', 'do_shortcode');
  }
  
  return;
  </code></pre>
  
  <p>and plugin.php's line from 418 to 434:</p>
  
  <pre><code>    // Sort
  if ( !isset( $merged_filters[ $tag ] ) ) {
      ksort($wp_filter[$tag]);
      $merged_filters[ $tag ] = true;
  }
  
  reset( $wp_filter[ $tag ] );
  
  do {
      foreach ( (array) current($wp_filter[$tag]) as $the_ )
          if ( !is_null($the_['function']) )
              call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
  
  } while ( next($wp_filter[$tag]) !== false );
  
  array_pop($wp_current_filter);
  </code></pre>
  
  <p>Any help would be appreciated.</p>