Passing flags to functions

I'd like to start a discussion on working out a generalized way of passing flags to internal PHP functions. I've often found myself writing a function and having to anticipate that it might need to accept some flags to affect its operation, and it was not immediately clear how those flags should be accessed. This is a bigger problem when you start working with functions that take optional arguments, because if you want to add an argument for flags you'll have to stick it at the end and make your users pass all intermediate arguments as well, non-required as they are. For example, I'd like to extend preg_replace to take a couple more flags that will its operation. Some of those flags are mutually exclusive, i.e. you can't specify two of them at the same time. So, what would be the way to pass them - using bitwise | (or)? This makes it hard to have groups of related flags that are mutually exclusive. Sticking them all into a 'flags array'? Seems a bit messy. Another question is how to parse those flags in a general way. I confess I don't have solutions currently, but that's why I wanted to initiate the discussion. Perhaps the answer lies in keyword arguments or somewhere else. -Andrei