PHP - Ordenar select por "last_name" y no por "id"

<select name="booking[agent_id]" class="os-form-control agent-selector">

  <?php foreach($agents as $agent){ ?>

    <option value="<?php echo $agent->id; ?>" <?php if($agent->id == $booking->agent_id) echo 'selected'; ?>><?php echo join(' ', array($agent->first_name, $agent->last_name)); ?></option>

  <?php } ?>

</select>