php 在字符串指定位置插入新字符的简单实现

$option = '<select class="sla_list">';
  
  // 取出所有策略
  $sla_query = $this->db->select('sla_id, name')->get('sla');
    $sla_res = $sla_query->result_array();
    if (!empty($sla_res)) {
      $option .= '<option value="0">未保护</option>';
      foreach ($sla_res as $k1 => $v1) {
        $option .= '<option value="' . $v1['sla_id'] . '">' . $v1['name'] . '</option>';
      }
      $option .= '</select>';

    } else {
      $option .= '<option value="0">未保护</option></select>';
    }


  // 选中默认的保护策略
  $vm_query = $this->db->select('sla_id')->where('vm_id', $vm_id)->get('task_vm', 1);
  $vm_res = $vm_query->row_array();
  if (is_null($vm_res['sla_id'])) $res['sla_id'] = 0;
  $selected = 'value="' . $vm_res['sla_id'] . '"';
  $new_str_arr = explode($selected, $option);
  $new_option = implode(" {$selected} selected", $new_str_arr);