php script to return files in folder as JSON from web service

getFilesAsJSON.php

<?php

/*
call this with GET
returns JSON of files in folder
optional arguments:
folder (defaults to current folder)
extension (defaults to all files)
 */

if (isset($_GET['folder'])) {
    $folder = $_GET['folder'];
    $thelot = scandir($folder);
}
else {
    $folder = './';
    $thelot = scandir($folder);
}

if (isset($_GET['extension'])) {
    $extension = $_GET['extension'];
    $justfiles = array_filter($thelot, function($item) use ($extension) {
        return !is_dir($item) and pathinfo($item, PATHINFO_EXTENSION) == $extension;
    });
}
else {
    $justfiles = array_filter($thelot, function($item) {
        return !is_dir($item);
    });
}

echo json_encode($justfiles);

?>