AssetConverter.php
public function convert($asset, $basePath)
    {
        if (($dotPos = strrpos($asset, '.')) === false)
            return $asset;

        if (($ext = substr($asset, $dotPos + 1)) !== self::INPUT_EXT)
            return parent::convert($asset, $basePath);

        $assetFilemtime = @filemtime("$basePath/$asset");
        $result = $this->buildResult($asset, $dotPos, ($this->cacheSuffix === true) ? $assetFilemtime : null);
        $resultFilemtime = @filemtime("$basePath/$result");

        if ($resultFilemtime < $assetFilemtime) {
            $this->parseLess($basePath, $asset, $result);
        }

        return $result;
    }
public function convert($asset, $basePath)
    {
        if (($dotPos = strrpos($asset, '.')) === false)
            return $asset;

        if (($ext = substr($asset, $dotPos + 1)) !== self::INPUT_EXT)
            return parent::convert($asset, $basePath);

        $assetFilemtime = @filemtime("$basePath/$asset");
        $result = $this->buildResult($asset, $dotPos, ($this->cacheSuffix === true) ? $assetFilemtime : null);
        $resultFilemtime = @filemtime("$basePath/$result");

        if ($resultFilemtime < $assetFilemtime) {
            $this->parseLess($basePath, $asset, $result);
        }

        return $result;
    }
Converts a given LESS assets file into a CSS

@param string $asset the asset file path, relative to $basePath
@param string $basePath the directory the $asset is relative to.
@return string the converted asset file path, relative to $basePath.