A placeholder is the displaying text when no value has not set. Simply call set_placeholder() method to set the placeholder text. When placeholder is used for a non-multi-value select box, it requires that you include an empty <option></option> tag as your first option.
Optionally, a clear button (visible once a selection is made) is available to reset the select box back to the placeholder value.
$pac = new C_PhpAutocomplete('country', $data);
$pac -> set_placeholder('Choose a country name:');
$pac -> display();
<input type="hidden" id="country">