How to Prevent SQL Injection in PHP

How can I prevent SQL injection in PHP?

The correct way to avoid SQL injection attacks, no matter which database you use, is to separate the data from SQL, so that data stays data and will never be interpreted as commands by the SQL parser. It is possible to create an SQL statement with correctly formatted data parts, but if you don't fully understand the details, you should always use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.

You basically have two options to achieve this:

Using PDO (for any supported database driver):

$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
$stmt->execute([ 'name' => $name ]);

foreach ($stmt as $row) {
    // Do something with $row
}

Using MySQLi (for MySQL):

$stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
$stmt->bind_param('s', $name); // 's' specifies the variable type => 'string'
$stmt->execute();

$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    // Do something with $row
}

If you're connecting to a database other than MySQL, there is a driver-specific second option that you can refer to (for example, pg_prepare() and pg_execute() for PostgreSQL). PDO is the universal option.

Correctly setting up the connection

PDO

Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:

$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8mb4', 'user', 'password');

$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

In the above example, the error mode isn't strictly necessary, but it is advised to add it. This way PDO will inform you of all MySQL errors by means of throwing the PDOException.

What is mandatory, however, is the first setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it to the MySQL server (giving a possible attacker no chance to inject malicious SQL).

Although you can set the charset in the options of the constructor, it's important to note that 'older' versions of PHP (before 5.3.6) silently ignored the charset parameter in the DSN.

Mysqli

For mysqli we have to follow the same routine:

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT); // error reporting
$dbConnection = new mysqli('127.0.0.1', 'username', 'password', 'test');
$dbConnection->set_charset('utf8mb4'); // charset

Explanation

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 an 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 of 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 (using PDO):

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

$preparedStatement->execute([ 'column' => $unsafeValue ]);

Can prepared statements be used for dynamic queries?

While you can still use prepared statements for the query parameters, the structure of the dynamic query itself cannot be parametrized and certain query features cannot be parametrized.

For these specific scenarios, the best thing to do is use a whitelist filter that restricts the possible values.

// Value whitelist
// $dir can only be 'DESC', otherwise it will be 'ASC'
if (empty($dir) || $dir !== 'DESC') {
   $dir = 'ASC';
}

prevent sql injection in mysqli

As already stated in comments, you need to be consistent with your API choice. You can't mix APIs in PHP.

You started out with mysqli_*, so I'll continue with that. You had some mysql_* and PDO in there, and it might not be a bad idea to use PDO over mysqli_* - but if your server supports mysqli_*, there is nothing wrong with using that. See Choosing an API and decide for yourself (just stay away from mysql_*, it's outdated).

Using mysqli_*, you connect to the database like this (you didn't show your connection).

$mysqli = new mysqli("host", "username", "password", "database");
if ($mysqli->connect_errno) {
    echo "Failed to connect to MySQL: (".$mysqli->connect_errno.") ".$mysqli->connect_error;
}
$mysqli->set_charset("utf8");

As for preventing SQL injection in it self, all you need is to use prepared statements. You can still clean or sanitize your data if there are some kind of values you don't want sitting in your tables - but that's kind of another discussion.

You also need to know if your passwords are hashed in the database. They really should be, and you should be using password_hash($password, $algorithm) and password_verify($password, $hash) if you're on PHP5.5 and above (if not, look into something like password_compat).

You need to be consistent with your hashes too, you can't insert it with md5 and selecting it with no hash. It all needs to be the same. Because if you are selecting an md5 hash, and comparing it to an unhashed string, they will be different, and the query fails.

I'm showing you an example of using password_verify(), so that means that the password stored in the database will also need to be stored with password_hash() (or your query fails).

if ($stmt = $mysqli->prepare("SELECT uid, password FROM users where email=?")) {
    $stmt->bind_param("s", $_POST['email']);           // Bind variable to the placeholder
    $stmt->execute();                                 // Execute query
    $stmt->bind_result($userID, $password);         // Set the selected columns into the variables
    $stmt->fetch();                                   // ...and fetch it
    if ($stmt->num_rows) {
        if (password_verify($_POST['password'], $password)) {
            // Password was correct and matched the email!
        } else {
            // Password was incorrect...
        }
    } else {
        // Accountname not found
    } 
}

This is just a basic example, but it will get you started. Never trust user input, use prepared statements.

PHP - Preventing SQL Injection on INSERT STATEMENT

Use PDO and prepared queries.Use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.Have a look at below example.

($conn is a PDO object)

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

$stmt = $conn->prepare("INSERT INTO users VALUES(:firstname, :lastname)");
$stmt->bindValue(':firstname', $firstname);
$stmt->bindValue(':lastname', $lastname);
$stmt->execute();

Prevent SQL injection attack in PHP

SQLite3::exec is for executing a query string, not a prepared statement. You need to use SQLite3Stmt::execute instead. Change:

if (!$db->exec ($sql)) {
    throw new Exception($db->lastErrorMsg());
}

if (!$sql->execute()) {
    throw new Exception($db->lastErrorMsg());
}

Note you can't echo $sql as it is an object, not a simple type. If you want to look at what a SQLite3Stmt object looks like, you would need to print_r($sql) or var_dump($sql).

How to prevent SQL injections in manually created queries?

I dont want to use other method

You should use whatever provides the required functionality, not the method that you like more over others!

Also you should never access superglobals directly in CakePHP, this will only bring you in trouble, especially in unit tests. User the proper abstracted methodes provided by the request object, that is CakeRequest::query().

Cookbook > Controllers > Request and Response objects > Accessing Querystring parameters

Use prepared statements

That being said, use prepared statements, either by passing the values to bind to the second argument of Model::query():

$result = $this->Search->query(
    "select * from subcategories where subcat_name like ? and subcat_status='active'",
    array('%' . $this->request->query('searchkey') . '%')
);

API > Model::query()

or by using DboSource::fetchAll(), which accepts parameters as the second argument too:

$db = $this->Search->getDataSource();
$result = $db->fetchAll(
    "select * from subcategories where subcat_name like ? and subcat_status='active'",
    array('%' . $this->request->query('searchkey') . '%')
);

Cookbook > Models > Retrieving Your Data > Prepared Statements
API > DboSource::fetchAll()

Escape manually

For the sake of completeness, it's also possible to manually escape the value via DboSource::value(), however you should avoid constructing query strings that way at all costs, as a small mistake can end up causing an unescaped value to be inserted, thus creating a possible SQL injection vulnerability:

$searchkey = $this->request->query('searchkey');

$db = $this->Search->getDataSource();
$value = $db->value('%' . $searchkey . '%', 'string');

$result = $this->Search->query(
    "select * from subcategories where subcat_name like $value and subcat_status='active'"
);

API > DboSource::value()

A PHP function to prevent SQL Injections and XSS

mysql_real_escape_string() doesn't prevent XSS. It will only make impossible to do SQL injections.

To fight XSS, you need to use htmlspecialchars() or strip_tags(). 1st will convert special chars like < to < that will show up as <, but won't be executed. 2nd just strip all tags out.

I don't recommend to make special function to do it or even make one function to do it all, but your given example would work. I assume.