IIRC, in the PHP3 / PHP4 days, the only database binding available in out-of-the-box PHP was MySQL.

I don't remember it like that. I think even php2 had support for more databases. One I am sure it supported was mSQL. I think they always had ms sql and oracle too. They were after all always very quick to provide bindings for anything with a C library. One of the reasons for php syntax being all over the place is because they tried to keep the C semantics for any given library.

Perhaps what you are remembering is a particular OS or installer's packaging.

You're right, it's there in PHP 2. https://web.archive.org/web/20000815061332/http://www.php.ne...

I think I'm remembering the Win32 binaries. At https://museum.php.net/win32/ the PHP3 binaries only have MySQL.

Shared hosts settling on MySQL probably made the difference.