Lua code

Textastic offers syntax highlighting for many languages including PHP, but it doesn't include compilers or interpreters due to restrictions of iOS.

You will notice that all apps that support running code - like Kodiak, Codea and Pythonista for example - do not have file download capabilities. So, if I wanted to add the ability to run code in Textastic, I'd have to remove FTP, "Open In", WebDAV, Dropbox and all other means of sharing files with other apps.

This is what the iOS Developer Program License Agreement says:

3.3.2 An Application may not download or install executable code. Interpreted code may only be used in an Application if all scripts, code and interpreters are packaged in the Application and not downloaded. The only exception to the foregoing is scripts and code downloaded and run by Apple's built- in WebKit framework, provided that such scripts and code do not change the primary purpose of the Application by providing features or functionality that are inconsistent with the intended and advertised purpose of the Application as submitted to the App Store.

These are the relevant points in the App Store review guidelines:

2.7 Apps that download code in any way or form will be rejected
2.8 Apps that install or launch other executable code will be rejected

That's a limitation that is imposed by Apple, sorry.