use Nikic\IncludeInterceptor\Interceptor;
$interceptor = new Interceptor(function(string $path) {
    if (!wantToIntercept($path)) {
        return null;
    }
    return transformCode(file_get_contents($path));
});
$interceptor->setUp(); // Start intercepting includes
require 'src/foo.php';
$interceptor->tearDown(); // Stop intercepting includes

use Nikic\IncludeInterceptor\Interceptor;
use Nikic\IncludeInterceptor\FileFilter;
$fileFilter = FileFilter::createAllWhitelisted();    // Start with everything whitelisted
$fileFilter->addBlackList(__DIR__ . '/src/');        // Blacklist the src/ directory
$fileFilter->addWhiteList(__DIR__ . '/src/foo.php'); // But whitelist the src/foo.php file
$interceptor = new Interceptor(function(string $path) use ($fileFilter) {
    if (!$fileFilter->test($path)) {
        return null;
    }
    return transformCode(file_get_contents($path));
});
$interceptor->setUp();
require 'src/foo.php';