There is no 'selected' attribute in php, checkboxes are not considered objects per se, but instead a single entity.
So, to retrieve your values would be via $_POST/$_GET['specialities']. The specialites index is a type of array and can be iterated through. Each value should correspond to a value that has been checked:
'specialities' => array(
 => 'bla',
 => 'bla bla'
So you can explode the values if you want: $vals = explode($_POST/_GET['specialities']);
Would be the value of the _REQUEST['specialties'] array if both of these options are selected. Be warned, if only one option is selected, the value will still be an array as you have specified in the html as it being such.