How to deal with different versions of PHP

Question

I'm writing a Wordpress plugin, so it's going to be used by different users and different PHP versions. The problem is that I found that some of the functions (like json_encode) are available in PHP 5.3 and not in PHP 5.2 or less. This creates a big issue, as most users don't have the latest version.

I want now, after getting the plugin 99% done, to do the following

1. Test my code with some kind of an app. where I can put the minimum PHP version. That App. or program would find out functions like json_encode. Not sure if that is possible, but would probably solve most of my problem.
2. Is it possible to get the native code of the PHP functions in PHP. I'm not sure if they are written in PHP or not, if so where can I get them. If not, what's the best option to find replacement for these functions. Certainly, I don't want to be re-coding them from scratch
3. What's the best methodology to implement the functions. I found some developers that check for the PHP version, while others check if the functions exist. Which one is best and why?

Would love also to read about your deployment strategies and how you dealt with that particular problem.

Answer 1

Re 1.) there may be such an application, but there also may be not. What you usually want to do is to keep close tabs on each function you use in the manual, which states what PHP version(s) are needed.

Re 2.) if a function is PHP 5.3 or 5.2 only, you will usually find a replacement suggestion in the User Contributed Notes or on Stack Overflow. (Be careful what you use though, a lot of the code in the UCN is bad - but in that case, there is usually at least one comment saying so.)

Re. 3.) it probably doesn't matter, but checking for whether a function exists is surely the safest way.