framework/console/controllers/AssetController.php
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}'.");
        }
    }
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}'.");
        }
    }
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.