Tried but can't write or read Chinese input to MySQL database?

Question

I've tried inserting and reading from my MySQL database, but it doesn't seem to work. Here's what I've read, and tried:

Other SO posts:

http://stackoverflow.com/questions/742205/mysql-alter-table-collation/5468980#5468980

http://stackoverflow.com/questions/5906585/change-database-collation

Tried changing collation to utf-unicode-ci and character set to UTF-8, didn't work

http://stackoverflow.com/questions/9401567/php-chinese-characters-cant-insert-to-mysql

This is more for insertion, but reading is equally as problematic.

Here are screenshots of what happened:

Insertion error

https://i.stack.imgur.com/2ot0L.png

Display error

https://i.stack.imgur.com/Bndz1.png

Table structure

https://i.stack.imgur.com/XO3K5.png

Code snippets for writing Chinese to database (registering a new user):

registration.js

var postData = {
    full_name   :   $("#full-name").val(),
    email       :   $("#email").val(),
    password    :   $("#password").val(),
    username    :   $("#username").val()
};

$.ajax({
    type        :   'POST',
    url         :   '../php/register.php', 
    data        :   postData,
    dataType    :   'JSON',
    success     :   function(data) {
        if (data.result == -1)
            alert('Registration unsuccessful!');
        else if (data.result == 0) {                // redirect to page
            alert('Registration successful!');
            window.location = '../home.php';
        }
    }
});

registration.php

$query = "  INSERT INTO `users`(`Username`, `Display_name`, `Password`, `Email`, `Date created`) 
            VALUES ('{$username}', '{$display_name}', '{$password}', '{$email}', '{$date}')";

if ( ($result = mysqli_query($link, $query)) == false ) 
    return -1;
else
    return 0;

Code for reading Chinese from database (displaying user name):

functions.php

$query = "SELECT display_name, followers FROM users ORDER BY followers DESC LIMIT 10";
$result = mysqli_query($link, $query);

while ($row = mysqli_fetch_array($result)) {
    $html = "<li>{$row['display_name']}<span id = 'popular-user-followers'>{$row['followers']}</span></li>";
    echo $html;
}

Would appreciate any advice on how to proceed, I'm not entirely familiar with databases yet. Thanks!