// Created on savesnippets.com · https://savesnippets.com/45Pzfh8KL7msRP
<?php
function walkFiles(string $root, ?callable $filter = null): Generator {
    $it = new RecursiveIteratorIterator(
        new RecursiveDirectoryIterator($root, FilesystemIterator::SKIP_DOTS)
    );
    foreach ($it as $file) {
        if (!$file->isFile()) continue;
        if ($filter && !$filter($file)) continue;
        yield $file->getPathname();
    }
}

// Every .php file under src/
foreach (walkFiles('src/', fn($f) => $f->getExtension() === 'php') as $path) {
    echo $path, PHP_EOL;
}