code: public function combineCssFiles($inputFiles, $outputFile)
      {
          $content = '';
          $outputFilePath = dirname($this->findRealPath($outputFile));
          foreach ($inputFiles as $file) {
              $content .= "/*** BEGIN FILE: $file ***/\n"
                  . $this->adjustCssUrl(file_get_contents($file), dirname($this->findRealPath($file)), $outputFilePath)
                  . "/*** END FILE: $file ***/\n";
          }
          if (!file_put_contents($outputFile, $content)) {
              throw new Exception("Unable to write output CSS file '{$outputFile}'.");
          }
      }
docs: Combines CSS files into a single one.
  @param array $inputFiles source file names.
  @param string $outputFile output file name.
  @throws \yii\console\Exception on failure.