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;}