Annotation not directly related to the mb_string vs. iconv conversion:
I wonder which implementation is faster for findings and/or non findings. This one or
the one that can be found @ http://stackoverflow.com/questions/834303/php-startswith-and-endswith-functions:

If the string really ends with the needle, it should be ok. But if it does not, rpos will likely search the complete string for the last occurrence.