If this is the case, another way of doing it would be:

$s='0102030405060A0BC'
# Fix to even number of chars
$s.=(length($s)%2?"F":"")
for($i=0; $i<length($s); $i+=2) {
  substr($s,$i,2)=reverse(substr($s,$i,2));
}
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jeffa: you were getting empty elements because the two-character pairs are being used as delimiters, with empty elements between them.

--ZZamboni