When developing the registration system, the database is connected. After clicking the registration button, the page is blank. What is the reason? code show as below

Question

<?php

session_start();

header("Content-type:text/html;charset=utf-8");

$link = mysqli_connect('localhost','000000','000000','000000');

if (!$link) {

die("Connection failed:".mysqli_connect_error() );

}

$sql = "select * from login";

$result = mysqli_query($link, $sql);

$ rows = mysqli_fetch_array($result);

$username = $_POST['username'];

$password = $_POST['password'];

$confirm = $_POST['confirm'];

$email = $_POST['email'];

if($username == "" || $password == "" || $confirm == "" || $email == "")

if ((strlen($username) < 3)||(!preg_match ('/^\w $/i', $username)))

{

echo "<script>alert('The information cannot be empty! Refill');window. location.href='zhuce.html'</script>";

} elseif ((strlen($username) < 3)||(!preg_match('/^\w $/i' , $username))) {

echo "<script>alert('Username must be at least 3 digits and does not contain illegal characters! Refill it');window.location.href='zhuce'</ script>";

//Determine the length of the username

}elseif(strlen($password) < 5){

echo "<script>alert(' The password must be at least 5 digits! Refill');window.location.href='zhuce.html'</script>";

//Judge password length

}elseif($password != $confirm) {

echo "<script>alert('The two passwords are different! Refill');window.location.href='zhuce.html'</script>";

//Check whether the password entered twice is the same

} elseif (!preg_match('/^[\w\.] @\w \.\w $/i', $email))

echo "<script>alert('The email is illegal! Refill');window.location.href='zhuce.html'</script>";

//Determine the email format Is it legal

elseif(mysqli_fetch_array(mysqli_query($link,"select * from login where username = '$username'"))){

echo "<script>alert('User name already exists');window.location.href='zhuce.html'</script>";

} else{

$sql= "insert into login(username, password , confirm, email)values('$username','$password','$confirm','$email')";

//Insert into the database

if(!(mysqli_query ($link,$sql))){

echo "<script>alert('Data insertion failed');";

}else{

echo "<script>alert('Registration successful');window.location.href='yonghu.html'</script>";

}

}

?>