upload.php

<?php
$static_root = "/tmp/";

function generate_name ($name) {
	do {
		$newname = '';
		for ($i = 5; $i > 0; $i--) {
			$newname .= base_convert(mt_rand(0, 36), 10, 36);
		}
		$newname .= strtolower(strrchr($name, '.'));
	} while (file_exists($newname));

	return $newname;
}

$out = array();
$files = $_FILES["files"];
$num_files = count($files["name"]);

for ($i = 0; $i < $num_files; $i++) {
	$newname = generate_name($files["name"][$i]);

	if ($files["error"][$i] == 0 && move_uploaded_file($files["tmp_name"][$i], $static_root . $newname)) {
		$out[] = array(
			"name" => $files["name"][$i],
			"url" => $newname
		);
	} else {
		http_response_code(500);
		echo json_encode(array(
			"success" => false
		));
	}
}
echo json_encode($out, JSON_PRETTY_PRINT);
?>