The mysqli_rollback() function in PHP is used to undo (rollback) a transaction that has been started using the mysqli_begin_transaction() function. When you perform multiple queries within a transaction, and something goes wrong, calling mysqli_rollback() will revert all changes made during the transaction and ensure data consistency.

Syntax:

mysqli_rollback(connection);

• connection: The MySQL connection object returned by mysqli_connect().

Example Usage:

1. Starting a Transaction: To begin a transaction, you use mysqli_begin_transaction() before running any queries. If an error occurs during the queries, you can call mysqli_rollback() to undo everything.

2. Rollback in Case of Error: If an error occurs during any query execution, you can rollback the changes to keep your data in a consistent state.

Example:

<?php
// Create a connection to the MySQL database
$mysqli = new mysqli("localhost", "username", "password", "database");

// Check if the connection was successful
if ($mysqli->connect_error) {
    die("Connection failed: " . $mysqli->connect_error);
}

// Start the transaction
$mysqli->begin_transaction();

try {
    // Query 1: Insert into table1
    $sql1 = "INSERT INTO table1 (name, email) VALUES ('John Doe', 'john@example.com')";
    if (!$mysqli->query($sql1)) {
        throw new Exception("Error in Query 1: " . $mysqli->error);
    }

    // Query 2: Insert into table2
    $sql2 = "INSERT INTO table2 (user_id, product) VALUES (LAST_INSERT_ID(), 'Product A')";
    if (!$mysqli->query($sql2)) {
        throw new Exception("Error in Query 2: " . $mysqli->error);
    }

    // If both queries succeed, commit the transaction
    $mysqli->commit();
    echo "Transaction successful!";
} catch (Exception $e) {
    // An error occurred, so rollback the transaction
    $mysqli->rollback();
    echo "Transaction failed: " . $e->getMessage();
}

// Close the connection
$mysqli->close();
?>

Explanation:

1. $mysqli->begin_transaction(): Starts the transaction.
2. Query 1: Attempts to insert data into table1.
3. Query 2: Attempts to insert data into table2 using LAST_INSERT_ID() to reference the last inserted ID from table1.
4. If any query fails (for example, if there’s an issue with the second insert), an exception is thrown.
5. $mysqli->rollback(): If an exception is caught, the changes are rolled back to the state before begin_transaction().
6. $mysqli->commit(): If all queries succeed, the changes are committed to the database.

When to Use:

• You use mysqli_rollback() when you need to ensure that a group of database queries are either fully committed or fully rolled back.
• This is useful when you are performing related database operations, and partial changes could result in data inconsistency.

Key Points:

• Commit: When everything goes as expected, you call mysqli_commit() to apply the changes.
• Rollback: If something goes wrong during the transaction, use mysqli_rollback() to revert the changes.