The current text encoding of that string is rather insubstantial. What you have there are HTML entities; they have little to do with the underlying "physical" encoding like ISO-8859 or UTF-8. What you want is to decode those HTML entities into a byte representation of the characters in a specific encoding, in this case to UTF-8. Therefore:
echo html_entity_decode('连衣裙', ENT_COMPAT, 'UTF-8');
// 连衣裙