<?php
//MYSQL连接操作
    $conn = mysql_connect("localhost","root","root") or die("数据库链接错误".mysql_error());
    mysql_select_db("db_database15",$conn) or die("数据库访问错误".mysql_error());
    mysql_query("set names gb2312");
?>
  <form id="form1" name="form1" method="post" action="">
    <table border="0" cellpadding="0" cellspacing="0">
      <tr>
            <td width="100" height="30" align="right" valign="middle" scope="col">用户名：</td>
        <td width="100" height="30" align="left" valign="middle" scope="col"><label for="textfield"></label>
        <input name="username" type="text" id="username" size="15" /></td>
        <td width="100" align="center" valign="middle" scope="col"><input type="submit" name="Submit" value="检查" id="Submit" /></td>
      </tr>
    </table>
  </form>
<?php
    if(trim($_POST[username]) != ""){    //trim去掉输入进来左右两边的空格
        $usr = crypt(trim($_POST[username]),"tm");    //用crypt()函数进行加密
        $sql = "select * from tb_user where user = '".$usr."'";    //mysql查询语句
        $rst = mysql_query($sql,$conn);    //执行MYSQL语句
        if(mysql_num_rows($rst) > 0){    //判断返回的行数是否大于0
            echo "<font color='red'>用户名已存在。</font>";
        }else{
            echo "<font color='green'>恭喜您：用户名可以使用!</font>";
        }
    }
?>