在写注册系统时，如果要检查用户名是否存在，php可以这样写：
$username=$_REQUEST['username'];
$password=$_REQUEST['password'];
echo "<br/>excute the insertUserInfo";
$con=mysqli_connect("localhost","root","","my_db");
$sql="SELECT * FROM user WHERE username='$username'";
ho "<br/>this username aready exists"; } else { $sql="INSERT INTO user (username,password) VALUES ('$username','$password')"; if(!mysqli_query($con,$sql)) { echo "<br/>Error".mysqli_error($con); } else { echo "<br/>one record added"; } }
建议不要用：
mysql_num_rows($result)</pre>
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