How to prevent sql injection in php

$unsafe_variable = $_POST['user_input'];
mysql_query("INSERT INTO table (column) VALUES ('" . $unsafe_variable . "')");

This is Because the user can enter something like VALUE "); DROP TABLE table; - , making the query become:

INSERT INTO table (column) VALUES('VALUE'); DROP TABLE table;'

How to avoid this situation? Use prepared statements and parameterized queries. SQL statements with any parameters will be sent to the database server and parsed! It is impossible for an attacker to maliciously inject SQL! There are two options: 1. Use PDO (PHP Data Objects):

$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
$stmt->execute(array(':name' => $name)) ;
foreach ($stmt as $row) {
// do something with $row
}

For a detailed introduction to using pdo to prevent sql injection, please refer to: Learn to use PDO to query Mysql to avoid SQL injection risk.

2. Use mysqli:

$stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
$stmt->bind_param('s', $name);
$stmt-> execute();
$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
// do something with $row
}

PDO (PHP Data Object) Note that when using PDO to access the mysql database, the real prepared statements are not used by default! To resolve this issue, you must disable emulation of prepared statements.

An example of using PDO to create a connection is as follows:

$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8', 'user', 'pass');
$dbConnection->setAttribute(PDO:: ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

The error mode ERRMODE is not strictly necessary in the above example, but it is recommended to add it. This method does not stop the script when a fatal error occurs. And give the developer a chance to catch any errors (when PDOException is thrown). The setAttribute() line is mandatory, it tells PDO to disable emulated prepared statements and use real prepared statements. This ensures that statements and values ​​are not parsed by PHP before being sent to the MySQL database server (there is no opportunity for an attacker to inject malicious SQL). Of course you can set the character set parameter in the constructor options, especially note that 'old' PHP versions (5.3.6) will ignore the character set parameter in the DSN.

Explanation What happens when the SQL prepared statement you pass is parsed and compiled by the database server? Tell the database engine what you want to filter by specifying characters (like a? or like :name in the above example). Then call execute to execute the combined prepared statement and the parameter value you specified.

The most important thing here is that the parameter value is combined with a precompiled statement, not with a SQL string. SQL injection works by deceptively creating a SQL script that includes a malicious string and sends it to the database. Therefore, By sending actual separate sql parameters, you will reduce the risk. When using prepared statements, any parameters you send, will only be treated as strings (although the database engine may do some parameter optimization, which of course may eventually is a number). In the above example, if the variable $name contains 'sarah';DELETE * FROM employees, the result will only be a search string "'sarah';DELETE * FROM employees", you will not get an empty surface.

Another benefit: if the same statement is executed multiple times in the same session, this will only be parsed and compiled once. Example (using PDO):

$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');
$preparedStatement->execute(array(':column' => $unsafeValue ));