Split haystack at nth occurrence of needle?

I've been RTFMing for about an hour and I can't find a string function to split haystack 'one<br>two<br>three<br>four<br>five' at the nth occurrence of needle '<br>'. strpos gives me the position of the first needle, and strrpos gives me the position of the last needle. But I'm looking for the position of one of the in-between needles, so that I can use substr() to trim everything from there on. What very obvious function was I unable to find in the manual? Thanks, - Brian