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The performance gain from this change depends on the text encoding and input string size. For very small strings, other overheads tend to swamp the performance gains to some extent, such that the speedup is less than 2x. For medium-length strings (~100 bytes or so), the speedup is typically around 2.5x. The greatest performance gains are for UTF-8 strings which have already been marked as valid (using the GC flags on the zend_string object); for those, the speedup is more than 10x in many cases. The previous implementation first converted the haystack and needle to wchars, then searched for matches between the two sequences of wchars. Because we use -1 as an error marker when converting to wchars, error markers from invalid byte sequences in the haystack would match error markers from invalid byte sequences in the needle, even if the specific invalid byte sequence was different. I am not sure whether this behavior is really desirable or not, but anyways, this new implementation follows the same behavior so as not to cause BC breaks.