Q:

Can str_replace be safely used on a UTF-8 encoded string if it's only given valid UTF-8 encoded strings as arguments?

PHP's str_replace() was intended only for ANSI strings and as such can mangle UTF-8 strings. However, given that it's binary-safe would it work properly if it was only given valid UTF-8 strings as arguments?
Edit: I'm not looking for a replacement function, I would just like to know if this hypothesis is correct.

A:

Yes. UTF-8 is deliberately designed to allow this and other similar non-Unicode-aware processing.
In UTF-8, any non-ASCII byte sequence representing a valid character always begins with a byte in the range \xC0-\xFF. This byte may not appear anywhere else in the sequence, so you can't make a valid UTF-8 sequence that matches part of a character.
This is not the case for older multibyte encodings, where different parts of a byte sequence are indistinguishable. This caused a lot of problems, for example trying to replace an ASCII backslash in a Shift-JIS string (where byte \x5C might be the second byte of a character sequence representing something else).

A:

It's correct because UTF-8 multibyte characters are exclusively non-ASCII (128+ byte value) characters beginning with a byte that defines how many bytes follow, so you can't accidentally end up matching a part of one UTF-8 multibyte character with another.
To visualise (abstractly):

a for an ASCII character
2x for a 2-byte character
3xx for a 3-byte character
4xxx for a 4-byte character

If you're matching, say, a2x3xx (a bytes in ASCII range), since a < x, and 2x cannot be a subset of 3xx or 4xxx, et cetera, you can be safe that your UTF-8 will match correctly, given the prerequisite that all strings are definitely valid UTF-8.
Edit: See bobince's answer for a less abstract explanation.

