Use strtr() when possible

[scribu]

When replacing strings of the same lenght, it's better to use strtr() instead of str_replace() because it's faster.

use strtr() for single character replacements

There are conflicting reports on which is faster, seems strtr() is a little faster only for very short strings but the difference is less than a microsecond.

let's close then?

I guess it's not worth the trouble.