Changing mysql_connect to mysqli causes function to fail - php

Question

To begin with, I am a novice when it comes to PHP and MySQL.
I have a MySQL table called levels that contains two columns: level_id and mapData. Some time ago, I wrote a piece of code using mysql_connect that takes user inputted level_id and fetches the corresponding mapData from the table, and the code works as intended. See below:
<?php
$mysql_host = 'localhost';
$mysql_user = 'username';
$mysql_password = "password";
$mysql_database = 'database';
if (!mysql_connect($mysql_host,$mysql_user,$mysql_password)||!mysql_select_db($mysql_database)){
die('Connection failed');
}
function get_map_data($level_id,$field){
$query = "SELECT $field FROM levels WHERE level_id='$level_id'";
if($query_run = mysql_query($query)){
if($query_result = mysql_result($query_run,0,$field)){
return $query_result;
}
}
}
$user_input = mysql_real_escape_string(base64_decode($_GET["level_id"]));
$sql = "SELECT level_id FROM levels WHERE level_id='$user_input'";
$result = mysql_query($sql);
if(mysql_num_rows($result) >0){
$mapData = get_map_data($user_input,'mapData');
print $mapData;
}else{
echo "0";
}
?>
In the case that the user inputs a level_id that does not exist in the database, instead of mapData, he will receive 0. Everything works! I, however, read that mysql_connect is deprecated as of PHP 5.5 and decided to switch from using it in my file to using mysqli:
$conn = new mysqli($mysql_host, $mysql_user, $mysql_password, $mysql_database);
if ($conn->connect_error) {
trigger_error('Database connection failed: ' . $conn->connect_error, E_USER_ERROR);
}
After changing to mysqli, my get_map_data function has stopped working. I have made certain that the mysqli connection works, but I am simply hitting my head against the wall in making my function work using it. How do I fix my get_map_data function so that it functions again using mysqli?

Answer 1

You can't mix mysqli and mysql_ functions. Furthermore, your mysqli initialization is creating an object, not a resource. Lastly, there is no mysqli_result() function, so therefore we need to emulate what you're trying to do. You wanted the first row 0 and then wanted a specific $field. Below should be what you want.
function get_map_data($level_id,$field){
$query = "SELECT $field FROM levels WHERE level_id='$level_id'";
if($query_run = $mysqli->query($query)){
//no mysqli->result() function, so we need to seek to the row.
$query_run->data_seek(0);
if($query_result = $query_run->fetch_assoc()){
return $query_result[$field];
}
}
}

Answer 2

your get_map_data function currently has query running using mysql_query you have to change that to mysqli_query in order to execute:
function get_map_data($level_id,$field){
$query = "SELECT $field FROM levels WHERE level_id='$level_id'";
if($query_run = mysqli_query($conn,$query)){
if($query_result = mysqli_result($query_run,0,$field)){
return $query_result;
}
}
}