Código HTML:
<form method="POST">
<select name="Profesionales[]" size="3" multiple="">
<optgroup label="Profesionales">
<option value="Berto">Berto</option>
<option value="Carlo">Carlo</option>
<option value="Danco">Danco</option>
</optgroup>
</select>
<button type="reset">Reestablecer</button>
<button type="submit">Actualizar</button>
</form>
__________________
Código PHP:
<?php print_r($_REQUEST) ?>