Best way to stop SQL Injection in PHP

Take the following code and tell me what to do.

// connect to the mysql database

$unsafe_variable = $_POST[“user-input”];

mysql_query(“INSERT INTO table (column) VALUES (’” . $unsafe_variable . “’)”);

// disconnect from the mysql database

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Use prepared statements. These are SQL statements that sent to and parsed by the database server separately from any parameters.

If you use PDO you can work with prepared statements like this:

$preparedStatement = $db->prepare(‘SELECT * FROM employees WHERE name = :name’);

$preparedStatement->execute(array(’:name’ => $name));

$rows = $preparedStatement->fetchAll();

What happens is that the SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute the prepared statement is combined with the parameter values you specify.

The important thing here is that the parameter values are combined with the compiled statement, not a SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters you limit the risk of ending up with something you didn’t intend. Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains ‘Sarah’; DELETE * FROM employees the result would simply be a search for the string “‘Sarah’; DELETE * FROM employees”, and you will not end up with an empty table.

Another benefit with using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.

Oh, and since you asked about how to do it for an insert, here’s an example:

$preparedStatement = $db->prepare(‘INSERT INTO table (column) VALUES (:column)’);

$preparedStatement->execute(array(’:column’ => $unsafeValue));