The rules of adding a PHP variable inside of any MySQL statement are plain and simple:

1. Use prepared statements

This rule covers 99% of queries and your query in particular. Any variable that represents an SQL data literal, (or, to put it simply – an SQL string, or a number) MUST be added through a prepared statement. No exceptions. A constant value can be put as is.

This approach involves four basic steps

• in your SQL statement, replace all variables with placeholders
• prepare the resulting query
• bind variables to placeholders
• execute the query

And here is how to do it with all popular PHP database drivers:

Adding data literals using mysqli

Starting from PHP 8.2 you can do the entire prepare/bind/execute sequence in one call:

$type="testing";
$reporter = "John O'Hara";
$sql = "INSERT INTO contents (type,reporter,description) VALUES ('whatever',?,?)";
$mysqli->execute_query($sql, [$reporter, $description]);

If your PHP version is old, then prepare/bind/execute has to be done explicitly:

$type="testing";
$reporter = "John O'Hara";
$sql = "INSERT INTO contents (type,reporter,description) VALUES ('whatever',?,?)";
$stmt = $mysqli->prepare($sql);
$stmt->bind_param("ss", $reporter, $description);
$stmt->execute();

The code is a bit complicated but the detailed explanation of all these operators can be found in my article, How to run an INSERT query using Mysqli, as well as a solution that eases the process dramatically.

For a SELECT query you can use the same method as above:

$reporter = "John O'Hara";
$result = $mysqli->execute_query("SELECT * FROM users WHERE name=?", [$reporter]);
$row = $result->fetch_assoc(); // or while (...)

but again, if your PHP version is old, you will need to go through prepare/bind/execute routine and also add a call to get_result() method, in order to get a familiar mysqli_result from which you can fetch the data the usual way:

$reporter = "John O'Hara";
$stmt = $mysqli->prepare("SELECT * FROM users WHERE name=?");
$stmt->bind_param("s", $reporter);
$stmt->execute();
$result = $stmt->get_result();
$row = $result->fetch_assoc(); // or while (...)

Adding data literals using PDO

$type="testing";
$reporter = "John O'Hara";
$sql = "INSERT INTO contents (type,reporter,description) VALUES ('whatever',?,?)";
$stmt = $pdo->prepare($sql);
$stmt->execute([$reporter, $description]);

In PDO, we can have the bind and execute parts combined, which is very convenient. PDO also supports named placeholders which some find extremely convenient.

2. Use white list filtering

Any other query part, such as SQL keyword, table or a field name, or operator – must be filtered through a white list.

Sometimes we have to add a variable that represents another part of a query, such as a keyword or an identifier (a database, table or a field name). It’s a rare case but it’s better to be prepared.

In this case, your variable must be checked against a list of values explicitly written in your script. This is explained in my other article, Adding a field name in the ORDER BY clause based on the user’s choice:

Unfortunately, PDO has no placeholder for identifiers (table and field names), therefore a developer must filter them out manually. Such a filter is often called a “white list” (where we only list allowed values) as opposed to a “black-list” where we list disallowed values.

So we have to explicitly list all possible variants in the PHP code and then choose from them.

Here is an example:

$orderby = $_GET['orderby'] ?: "name"; // set the default value
$allowed = ["name","price","qty"]; // the white list of allowed field names
$key = array_search($orderby, $allowed, true); // see if we have such a name
if ($key === false) { 
    throw new InvalidArgumentException("Invalid field name"); 
}

Exactly the same approach should be used for the direction,

$direction = $_GET['direction'] ?: "ASC";
$allowed = ["ASC","DESC"];
$key = array_search($direction, $allowed, true);
if ($key === false) { 
    throw new InvalidArgumentException("Invalid ORDER BY direction"); 
}

After such a code, both $direction and $orderby variables can be safely put in the SQL query, as they are either equal to one of the allowed variants or there will be an error thrown.

The last thing to mention about identifiers, they must be also formatted according to the particular database syntax. For MySQL it should be backtick characters around the identifier. So the final query string for our order by example would be

$query = "SELECT * FROM `table` ORDER BY `$orderby` $direction";