Why Do I Get a 'Commands Out of Sync' Error in PHP with MySQL Queries?

"Commands Out of Sync" Error in PHP Code with MySQL Queries

When executing a PHP script that involves multiple MySQL queries using mysqli, an error can occur: "Commands out of sync; you can't run this command now." This issue arises when subsequent queries are attempted before fetching and processing the results from the previous query or clearing the MySQL server's cache of query commands.

The code provided in the question executes two queries: one to count records and another to retrieve rows based on a search string. However, before the results of the first query are fetched, the second query is executed. This triggers the "out of sync" error.

Causes of the Error

• Unbuffered Queries: MySQLI uses unbuffered queries by default for prepared statements, meaning results are streamed in parts as they are available. If a subsequent query is attempted before buffering the first query's results, commands can become out of sync and lead to the error.
• Incomplete Query Execution: When multiple queries are attempted without properly handling the results, incomplete commands can accumulate in the server's cache. This can disrupt subsequent queries and cause the error.

Possible Solutions

• Buffer Results: To avoid this error, use $stmt->store_result() to buffer the results of the query before executing the next one. Fetch the buffered results using $stmt->fetch() and then clear the server's cache with $stmt->free_result().
• Fetch First Query Results: Alternatively, fetch all results from the first query into an array. This consumes the results and allows subsequent queries to be executed without interference from unfetched results.
• Use Transactions: If multiple queries are related and must be executed sequentially, consider using transactions to handle the process as a single atomic unit.

Example

Here's an example of buffering the first query's results using $stmt->store_result():

<?php
$con = mysqli_connect("localhost", "user", "password", "db");
$brand ="o";
$countQuery = "SELECT ARTICLE_NO FROM AUCTIONS WHERE upper(ARTICLE_NAME) LIKE % ? %";
$con->query("SET NAMES 'utf8'");
if ($numRecords = $con->prepare($countQuery)) {
    $numRecords->bind_param("s", $brand);
    $numRecords->execute();
    $numRecords->store_result(); // Buffer the results
    $data = $con->query($countQuery) or die(print_r($con->error));
    $rowcount = $data->num_rows;
}

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