Find when Internal (built-in) Functions and Predefined Constants were introduced (version number) in PHP

Question

I was working on a couple of backwards compatibility issues and have been using the PHP manual for reference. Ive been using the version information located at the top of the page. Example, is_null version information is

(PHP 4 >= 4.0.4, PHP 5, PHP 7)

From this I have understood that is_null exists in PHP 4 only if the version is greater than or equal 4.0.4 and it also exists in all versions of PHP 5 & PHP 7.

I have come to realise that the information might not be correct as is_dir version information is

(PHP 4, PHP 5, PHP 7)

but by doing a little googling ("is_dir" "PHP 3") I found out it was introduced in version 3.

Firstly have I understood the version information correctly? and is there another way to find when a function or pre-defined constant was added ?

Edit

The PHP 4 Change Log and the PHP 5 Change Log were helpful, they provide a detailed list of all changes and I have no reason to believe that any information is incorrect. Unfortunately there doesn't seem to be a change log for anything prior to that.

Update

Although no change logs seems to exist, the source code for all versions of PHP are available to download from http://museum.php.net/ so it is possible to determine the origin of functions,predefined constants and any other part of PHP from viewing the source.

Answer 1

The php.net website is not a complete historic reference, it attempts to show you what should be relevant.

At some time in the not too distant future, I imagine a lot of the PHP4 stuff will be gone, since there's no sense in talking about PHP4 in the year 2015, and room must be made for PHP7.

There is even less sense in talking about PHP3, but it so happens that you can find the PHP3 manual in PDF form with a google search, here's the top result for me.