<?php
 $directory="/data";
 $files = scandir($directory);
 foreach($files as $key=>$name){
    $oldName = $name;
    $newName = strtolower($name);
    rename("$directory/$oldName","$directory/$newName");
  }
?>

<?php
$path = realpath('your/path/here');

$di = new RecursiveIteratorIterator(
    new RecursiveDirectoryIterator($path, FilesystemIterator::SKIP_DOTS),
    RecursiveIteratorIterator::LEAVES_ONLY
);

foreach($di as $name => $fio) {
    $newname = $fio->getPath() . DIRECTORY_SEPARATOR . strtolower( $fio->getFilename() );
    echo $newname,"

";
    //rename($name, $newname); - first check the output, then remove the comment...
}