SlabPHP File and Resource Concatenator, used in conjunction with the SlabPHP media concatenator controller.
composer require slabphp/concatenator
Create a manager:
$manager = new \Slab\Concatenator\Manager();
Set it up for where to look for files:
$manager
->setFileSearchDirectories([__DIR__ . '/resources'])
->addObject('css/test.css', [])
->addObject('css/something.css', [])
->concatenateObjectList();
Then just concatenate it:
$output = $manager->getOutput();