Where to put odds & ends PHP functions?

[eluser]Unknown[/eluser]
I'm using a pre-PHP 5.2 version of PHP and need json_encode. I have a replacement function in a PHP file, but need to know where this type of thing is meant to go to hook into my app.

I realize I could put it anywhere and include the file, but is there a specific location for these things to go?

Thanks in advance!