yii\web\AssetManager::getConverter PHP Method

    public function getConverter()
    {
        if ($this->_converter === null) {
            $this->_converter = Yii::createObject(AssetConverter::className());
        } elseif (is_array($this->_converter) || is_string($this->_converter)) {
            if (is_array($this->_converter) && !isset($this->_converter['class'])) {
                $this->_converter['class'] = AssetConverter::className();
            }
            $this->_converter = Yii::createObject($this->_converter);
        }
        return $this->_converter;
    }

 /**
  * Publishes the asset bundle if its source code is not under Web-accessible directory.
  * It will also try to convert non-CSS or JS files (e.g. LESS, Sass) into the corresponding
  * CSS or JS files using [[AssetManager::converter|asset converter]].
  * @param \yii\web\AssetManager $am the asset manager to perform the asset publishing
  */
 public function publish($am)
 {
     if ($this->sourcePath !== null && !isset($this->basePath, $this->baseUrl)) {
         list($this->basePath, $this->baseUrl) = $am->publish($this->sourcePath, $this->publishOptions);
     }
     $converter = $am->getConverter();
     foreach ($this->js as $i => $value) {
         if (is_array($value)) {
             if (isset($value['file'])) {
                 $js = $value['file'];
             } else {
                 continue;
             }
         } else {
             $js = $value;
         }
         if (strpos($js, '/') !== 0 && strpos($js, '://') === false) {
             if (isset($this->basePath, $this->baseUrl)) {
                 $js = $converter->convert($js, $this->basePath);
             } else {
                 $js = '/' . $js;
             }
             if (is_array($value)) {
                 $value['file'] = $js;
             } else {
                 $value = $js;
             }
             $this->js[$i] = $value;
         }
     }
     foreach ($this->css as $i => $value) {
         if (is_array($value)) {
             if (isset($value['file'])) {
                 $css = $value['file'];
             } else {
                 continue;
             }
         } else {
             $css = $value;
         }
         if (strpos($css, '/') !== 0 && strpos($css, '://') === false) {
             if (isset($this->basePath, $this->baseUrl)) {
                 $css = $converter->convert($css, $this->basePath);
             } else {
                 $css = '/' . $css;
             }
             if (is_array($value)) {
                 $value['file'] = $css;
             } else {
                 $value = $css;
             }
             $this->css[$i] = $value;
         }
     }
 }