<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>