PHP MySQLi \'Commands Out of Sync\' Error: How to Fix It?

PHP Commands Out of Sync Error: Understanding and Resolution

When executing multiple prepared statements in PHP/MySQLi, you may encounter the "Commands out of sync" error. This arises from a mismatch between the number of statements executed and the number of results being processed. To address this issue, consider the following insights and solutions:

Cause of the Error

The "Out of Sync" error occurs when the MySQL client receives more data than expected for a particular statement. This happens when using mysqli::query with the MYSQLI_USE_RESULT flag. This flag instructs MySQL not to fully buffer the results, causing subsequent commands to be out of sync.

Code Snippet Explanation

In your provided code, two prepared statements are used to retrieve data from the database. The first statement retrieves user information, while the second retrieves privileges. However, you're encountering the error while executing the second statement.

Resolution

To resolve the error, you must free the results of the first statement before executing the second. This is achieved by calling the mysqli_stmt->free_result() function. Additionally, invoking mysqli->next_result() after each query ensures that any additional results have been processed.

Here's an example of how to resolve the issue:

    $stmt = $mysqli->prepare("SELECT id, username, password, firstname, lastname, salt FROM members WHERE email = ? LIMIT 1");
    $stmt->bind_param('s', $loweredEmail);
    $stmt->execute();
    $stmt->store_result();
    $stmt->bind_result($user_id, $username, $db_password, $firstname, $lastname, $salt);
    $stmt->fetch();

    $stmt->free_result();
    $mysqli->next_result(); // Ensure no stray results are present

    $stmt1 = $mysqli->prepare("SELECT privileges FROM delegations WHERE id = ? LIMIT 1");
    $stmt1->bind_param('s', $user_id);
    $stmt1->execute();
    $stmt1->store_result();
    $stmt1->bind_result($privileges);
    $stmt1->fetch();

Additional Tips

• Use separate objects for each prepared statement to avoid confusion.
• Always check for errors after executing a statement using mysqli_stmt->error.
• Avoid using MYSQLI_USE_RESULT unless necessary, as it can lead to synchronization issues.

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