php连接数据库实现登录效果显示失败

Question

环境是用phpstudy_pro这个，软件是用的phpstorm,数据库是localhost的

点登录后，显示： 登录失败

<?php
header("Content-Type:text/html;charset=utf-8");
$username = trim($_POST['username']);
$pw = trim($_POST['pw']);
//进行必要的验证
if (!strlen($username) || !strlen($pw) ){
    echo"<script>alert('用户名和密码都必须要填写');history.back();</script>";
    exit;
}else {
    if (!preg_match('/^[a-zA-Z0-9]{3,10}$/', $username)) {
        echo "<script>alert('用户名必填，且只能由大小字符和数字构成，长度为3到10个字符！');history.back();</script>";
        exit;
    }
    if (!preg_match('/^[a-zA-Z0-9_*]{6,10}$/', $pw)) {
        echo "<script>alert('密码必填，且只能大小写字符和数字，以及*、_构成,长度为6-10个字符！');history.back();</script>";
        exit;
    }
}
//连接数据库
//第一步，连接到数据库
$conn = mysqli_connect("localhost","root","20031022","member");

//第二步，设置字符串
mysqli_query($conn,"set names utf8");

$sql = "select * from info where username = '".$username."' and pw ='“.md5($pw).“'";
$result = mysqli_query($conn,$sql);
$num = mysqli_num_rows($result);
if($num){
    echo "<script>alert('登录成功！')</script>";
}else{
    echo "<script>alert('登录失败！');history.back();</script>";
}

Answer 1

$sql = "select * from info where username = '".$username."' and pw ='“.md5($pw).“'";
.md5($pw) 两边是中文引号了。