PHP实现根据密码长度显示安全条

//根据密码长度显示安全条
          <ul class="clear">
          <li>密　　码：</li>
          <li> <input type="password" id="pwd" name="pwd" class="in" onkeyup=pwstrength(this.value) onblur=pw_y("pwd","pwd1")></li>
          <li><em class="red">*</em></li>
          <li class="i2 grey"><table border="0" bordercolor="#cccccc" style='display:marker'>
          <tr align="center">
            <td id="strength_l" bgcolor="#eeeeee">弱</td>
            <td id="strength_m" bgcolor="#eeeeee">中</td>
            <td id="strength_h" bgcolor="#eeeeee">强</td>
          <td align="left" >
           <label id="pwd1">使用数字，字母，下划线，长度在 6 - 20 个字符之间</label></td>
            </tr>
          </table>         
          </li>
          </ul>
function pwstrength(pwd){
  o_color="#eeeeee";
  l_color="#ff0000";
  m_color="#ff9900";
  h_color="#33cc00";
  if (pwd==null||pwd==''){
    lcolor=mcolor=hcolor=o_color;
  }else{
    s_level=checkstrong(pwd);
    switch(s_level) {
      case 0:
        lcolor=mcolor=hcolor=o_color;
      case 1:
        lcolor=l_color;
        mcolor=hcolor=o_color;
        break;
      case 2:
        lcolor=mcolor=m_color;
        hcolor=o_color;
        break;
      default:
        lcolor=mcolor=hcolor=h_color;
      }
  }
  document.getelementbyid("strength_l").style.background=lcolor;
  document.getelementbyid("strength_m").style.background=mcolor;
  document.getelementbyid("strength_h").style.background=hcolor;
  return;
}