while (start_ptr < end_ptr) {
    start_ptr += UTF8SKIP(start_ptr);
    len++;
}

##</code><code>##

PL_utf8_length($chunk)                    # first iteration
PL_utf8_length($chunk . $chunk)           # second iteration
PL_utf8_length($chunk . $chunk . $chunk)  # third iteration
...and so on...

##</code><code>##

PL_utf8_length($chunk)  # 1st
PL_utf8_length($chunk)  # 2nd
PL_utf8_length($chunk)  # 3rd
...