PocketMine-MP, a PHP-based application designed to create Minecraft: Pocket Edition servers, is free and available to download on GitHub.
Installing the software allows users to host a Minecraft: Pocket Edition server and share their blocky worlds as Minecraft players do on PC. According to an FAQ on GitHub, users must own the full version of Minecraft: Pocket Edition and can configure the software for use on a local network or publish it to a server to share online.
You can find information on how to set up and update a server on Windows, Mac and Linux here. PocketMine-MP also supports a PHP plugin API that "that enables a developer to extend it and add new features, or change default ones," according to the software's website.