Prepared Statements

Prepared statements are a feature used in database programming to execute the same SQL statement repeatedly with high efficiency and to secure the database against [[SQL injection]] attacks. In the context of [[PHP]] and databases like [[MySQL]], prepared statements are particularly important.

First, the SQL statement template is sent to the database server. This template contains placeholders (usually question marks ?) for parameters instead of actual values.

For instance: INSERT INTO users (username, email) VALUES (?, ?).

The database server parses, compiles, and performs query optimization on this SQL statement template, creating a prepared statement. At this point, the server understands what the query is meant to do and knows how to execute it efficiently, but it doesn't have the specific values for the placeholders yet.

Next, the application supplies the actual values for these placeholders. This binding of parameters can be done separately from the statement preparation. Importantly, these values are treated as data only, not as part of the SQL command.

The server then executes the prepared statement with the bound values. Since the SQL command structure is already known and optimized, the server can execute the statement more efficiently. If the same statement needs to be executed again, it can be done with new values without the need to recompile the SQL statement.

The most significant advantage of prepared statements is increased security. Since parameter values are sent later and treated distinctly from the SQL command text, SQL injection attacks are largely mitigated. Attackers cannot manipulate the statement structure through the data input.

An example of a prepared statement:

// Create a prepared statement
$stmt = $mysqli->prepare("INSERT INTO users (username, email) VALUES (?, ?)");

// Bind parameters (s for string, i for integer, etc.)
$stmt->bind_param("ss", $username, $email);

// Set parameters and execute
$username = "exampleUser";
$email = "user@example.com";
$stmt->execute();

// Repeat with different data
$username = "anotherUser";
$email = "another@example.com";
$stmt->execute();

$stmt->close();