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="#" 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;
}