First, some context (stuff that most of you know anyway):

Every popular programming language has a clear evolution, most of the time marked by its version: you have Java 5, 6, 7 etc., PHP 5.1, 5.2, 5.3 etc. Releasing a new version makes new APIs available, fixes bugs, adds new features, new frameworks etc. So all in all: it's good.

But what about the language's (or platform's) problems? If and when there's something wrong in a language, developers either avoid it (if they can) or they learn to live with it.

Now, the developers of those languages get a lot of feedback from the programmers that use them. So it kind of makes sense that, as time (and version numbers) goes by, the problems in those languages will slowly but surely go away. Well, not really. Why? Backwards compatibility, that's why. But why is this so? Read below for a more concrete situation.

The best way I can explain my question is to use PHP as an example:

PHP is loved thousands of people and hated by just as many thousands. All languages have flaws, but apparently PHP is special. Check out this blog post. It has a very long list of so called flaws in PHP. Now, I'm not a PHP developer (not yet), but I read through all of it and I'm sure that a big chunk of that list are indeed real issues. (Not all of it, since it's potentially subjective).

Now, if I was one of the guys who actively develops PHP, I would surely want to fix those problems, one by one. However, if I do that, then code that relies on a particular behaviour of the language will break if it runs on the new version. Summing it up in 2 words: backwards compatibility.

What I don't understand is: why should I keep PHP backwards compatible? If I release PHP version 8 with all those problems fixed, can't I just put a big warning on it saying: "Don't run old code on this version !"?

There is a thing called deprecation. We had it for years and it works. In the context of PHP: look at how these days people actively discourage the use of the mysql_* functions (and instead recommend mysqli_* and PDO). Deprecation works. We can use it. We should use it. If it works for functions, why shouldn't it work for entire languages?

Let's say I (the developer of PHP) do this:

• Launch a new version of PHP (let's say 8) with all of those flaws fixed
• New projects will start using that version, since it's much better, clearer, more secure etc.
• However, in order not to abandon older versions of PHP, I keep releasing updates to it, fixing security issues, bugs etc. This makes sense for reasons that I'm not listing here. It's common practice: look for example at how Oracle kept updating version 5.1.x of MySQL, even though it mostly focused on version 5.5.x.
• After about 3 or 4 years, I stop updating old versions of PHP and leave them to die. This is fine, since in those 3 or 4 years, most projects will have switched to PHP 8 anyway.

My question is: Do all these steps make sense? Would it be so hard to do? If it can be done, then why isn't it done?

Yes, the downside is that you break backwards compatibility. But isn't that a price worth paying ? As an upside, in 3 or 4 years you'll have a language that has 90 % of its problems fixed.... a language much more pleasant to work with. Its name will ensure its popularity.

EDIT: OK, so I didn't expressed myself correctly when I said that in 3 or 4 years people will move to the hypothetical PHP 8. What I meant was: in 3 or 4 years, people will use PHP 8 if they start a new project.