First off, I'm not quite sure what you're asking here. I don't recognize any between or betwixt type things in PHP.
If it's a PHP question, it needs to go in the PHP section, not general chat.
If it's a weirdly phrased grammer question, ye old American English generally uses "between." "Betwixt" is reserved for poems and witches.
"Betwixt" and "between" are, essentially, synonyms, but "betwixt" is now considered archaic by many people, or more useful for other contexts. When describing PHP code, "between" would be better.
Since this was a grammar question, here is another important thing about "between": be sure to use it only when there are only two items in question. When there are more than two, use "among" instead. Americans very often make the mistake of saying something like, "The prize money was divided evenly between Mark, John, and Jim," when it should be "The prize money was divided evenly among Mark, John, and Jim."
Betwixt is something you'd only hear/see in old literature. No one uses it in common day speech.
Yea, actually betwixt is an Archaic English word for Between. You'll find it in writings of the 1800s.
As far as I know betwixt is only used nowadays as part of the expression "betwixt and between" which means "neither one thing or the other", though even this expression does not seem to be used as much now as a few years ago.