php verify name exists in mysql

#How does php verify whether the name exists in mysql?

PHP connects to MySQL to query whether the user name exists (login verification)

The code is as follows:

query("select username from user where username ='{$username}' and password = '{$password}'"); //查询数据库中的用户名和密码 并返回集合
$row = mysqli_fetch_assoc($sql); //取其中一行
if ($row > 0) { //判断是否存在
echo "欢迎'{$username}'再次登录";
} else {
echo "，您还没有账号，请先进行注册！！！";
header("location:register.html"); //跳转至注册页面
}
?>

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