Secure php and sql when selecting and inserting data - php

Question

I have an app that takes data from MySQL database and also inserting data into it (the user is writing the data that is getting inserted) and honestly I am pretty new to php and don't know a lot about securing and sanitizing strings,
I want to make the php files more secure and I don't know what to look for in order of doing it, if someone can send a tutorial it will be great.
here is the select and insert codes
<?php
header('Content-Type: text/html; charset=utf-8');
$db = "*********";
$username = "*********";
$password = "*******";
$host = "************";
$sql = "select * from sample;";
$conn = mysqli_connect($host,$username,$password,$db);
$conn->set_charset('utf8');
$result = mysqli_query($conn,$sql);
$response = array();
while($row = mysqli_fetch_array($result))
{
array_push($response,array($row[0],$row[1],$row[2]));
}
$str = json_encode(array($response),JSON_UNESCAPED_UNICODE);
$str = clean($str);
echo $str;
mysqli_close($conn);
function clean($string) {
$string = str_replace(' ', ' ', $string);
$string = preg_replace('/[^a-zA-Z0-9,×-×–, : . -]/', '', $string);
return preg_replace('/-+/', '-', $string);
}
?>
and the insert:
<?php
$db = "*********";
$username = "*********";
$password = "*******";
$host = "************";
$conn = mysqli_connect($server_name,$mysql_username,$mysql_password,$db_name);
$name =$_POST["name"];
$publisher=$_POST["publisher"];
$date=$_POST["date"];
$sql_query = "insert into sample(name,publisher,date)
values('$name','$publisher','$date');";
if(mysqli_query($conn,$sql_query))
{
echo "data inserted";
}
else
{
echo "error";
}
?>

Answer 1

Use prepared statements any time possible:
$sql_query = "insert into sample(name,publisher,date) values(?,?,?);";
$stmt = mysqli_prepare($conn,$sql_query);
mysqli_stmt_bind_param( $stmt , "sss" , $name,$publisher,$date);
mysqli_stmt_execute($stmt);
And try to use the object style only, not the procedural of the mysqli extention.
You are mixing both here:
$conn = mysqli_connect($host,$username,$password,$db);//procedural style
$conn->set_charset('utf8');//oject style

Answer 2

You can use PDO. It's very simple to build safe SELECT and INSERT queries. Although, you must be careful on some commands such as ORDER BY.
<?php
$pdo = new PDO('mysql:host=localhost;dbname=databasename;charset=utf8', 'username', 'password');
$statement = $pdo->prepare("SELECT * FROM users WHERE firstname = :firstname AND lastname = :lastname");
$statement->execute(array(':firstname' => 'Max', ':lastname' => 'Mustermann'));
if( $statement->rowCount() > 0 ) {
$row = $statement->fetch();
echo "Hello " . $row['firstname'];
}
?>
Mysqli can be used too, but please check out mysqli_real_escape_string.