php无法添加数据怎么办

<?php

if( @ $_POST["登入注册"]=="登入" )
{

$servername = "localhost";
$username = "root";
$passWord = "123456";
$dbname = "student";
// 创建连接
$conn = new Mysqli($servername, $username, $password,$dbname);

mysqli_set_charset($conn,'utf8');
$x =$_POST["用户名"];
$y =$_POST["密码"];
$Z =$_POST["性别1"];
$O =$_POST["学历1"];
$a=mysqli_query($conn,"INSERT INTO student.plus1 (用户名, 密码, 性别, 学历) VALUES ('$x','$y','$Z','$O')");
if (mysqli_query($conn, $a)) {
echo "新记录插入成功";
} else {
echo "Error: " . $a . "<br>" . mysqli_error($conn);
}
mysqli_close($conn);
}
?>

$a=mysqli_query($conn,"INSERT INTO student.plus1 (用户名, 密码, 性别, 学历) VALUES ('$x','$y','$Z','$O')");