Keep strings outside of source files

Hello,

this is just a discussion about some practices, I would like to know how you do this and why. So, here is what is this topic about!

While programming single language application, we can just write strings directly into the source files. There will be something like:

$this->flashMessage("You have been signed out.");

…and the same for templates. Okay, sometimes we need multi language application. Now, I saw two solutions:

// We will have some default language, like english.
$this->flashMessage($this->translator->translate("You have been signed out."));

// We will have something like constants (named strings)
$this->flashMessage($this->translator->translate("SIGN_OUT_MESSAGE"));

// Or we can overwrite the flashMessage() implementation and do the localization there, this doesn't matter now.
$this->flashMessage("SIGN_OUT_MESSAGE");

So, when single application is made, we have no localization and messages for users are in source files. I guess, this is wrong, since every change or typo must be done by programmer who knows the source files. Of course, this doesn't matter in our own application.

This leads to using some kind of translator, which makes changes possible. However – we should not use default language in sources. Why? Imagine there is a typo in the default language message in sources. How administrator solves this? Does he translate the string?

Better would be using a constant or name for every used message. This is quite good for saving into database or a file and the only thing I don't like about this is the way it shows in editor – would be great if IDE displayed the assigned string.

Those things are already solved in Android Development Tools. I like the way it works – you have some file with strings (in XML, JSON, …) and the generator creates a real Java class with constants, for PHP, this what it would look like:

class R {
	const SIGN_IN = 'Login';
	const SIGN_OUT = 'Logout';
}

And, the IDE displays the string instead of identifier.

How do you solve this? I didn't find some kind of externalization of strings for PHP anywhere. One would say this is useless, but I don't think so – many messages or labels are used multiple times and what if we want, for instance, rename a feature of our application?