Causes of MySQL error 2014 Cannot execute queries while other unbuffered queries are active

The MySQL client protocol doesn't allow more than one query to be "in progress." That is, you've executed a query and you've fetched some of the results, but not all -- then you try to execute a second query. If the first query still has rows to return, the second query gets an error.

Client libraries get around this by fetching all the rows of the first query implicitly upon first fetch, and then subsequent fetches simply iterate over the internally cached results. This gives them the opportunity to close the cursor (as far as the MySQL server is concerned). This is the "buffered query." This works the same as using fetchAll(), in that both cases must allocate enough memory in the PHP client to hold the full result set.

The difference is that a buffered query holds the result in the MySQL client library, so PHP can't access the rows until you fetch() each row sequentially. Whereas fetchAll() immediately populates a PHP array for all the results, allowing you access any random row.

The chief reason not to use fetchAll() is that a result might be too large to fit in your PHP memory_limit. But it appears your query results have just one row anyway, so that shouldn't be a problem.

You can closeCursor() to "abandon" a result before you've fetched the last row. The MySQL server gets notified that it can discard that result on the server side, and then you can execute another query. You shouldn't closeCursor() until you're done fetching a given result set.

Also: I notice you're executing your $stmt2 over and over inside the loop, but it will return the same result each time. On the principle of moving loop-invariant code out of the loop, you should have executed this once before starting the loop, and saved the result in a PHP variable. So regardless of using buffered queries or fetchAll(), there's no need for you to nest your queries.

So I would recommend writing your code this way:

$sql ='SELECT temp_id FROM temp1';
$stmt2 = db::db()->prepare($sql);
$stmt2->execute();
$rs2 = $stmt2->fetchAll(PDO::FETCH_ASSOC);
$stmt2->closeCursor();

$sql='SELECT COUNT(*) AS valid FROM cities_has_zipcodes 
      WHERE cities_id=:cities_id AND zipcodes_id=:zipcodes_id';
$stmt1 = db::db()->prepare($sql);

foreach($data AS $row)
{
    try
    {
        $stmt1->execute($row);
        $rs1 = $stmt1->fetchAll(PDO::FETCH_ASSOC);
        $stmt1->closeCursor();
        syslog(LOG_INFO,'$rs1: '.print_r($rs1[0],1).' '.rand());
        syslog(LOG_INFO,'$rs2: '.print_r($rs2[0],1).' '.rand());
    }
    catch(PDOException $e){echo(sql_error($e));}            
}

Note I also used named parameters instead of positional parameters, which makes it simpler to pass $row as the array of parameter values. If the keys of the array match the parameter names, you can just pass the array. In older versions of PHP you had to include the : prefix in the array keys, but you don't need that anymore.

You should use mysqlnd anyway. It has more features, it's more memory-efficient, and its license is compatible with PHP.

I am hoping for a better answer than the following. While some of these solutions might "fix" the problem, they don't answer the original question regarding what causes this error.

1. Set PDO::ATTR_EMULATE_PREPARES=>true (I don't wish to do this)
2. Set PDO::MYSQL_ATTR_USE_BUFFERED_QUERY (didn't work for me)
3. Use PDOStatement::fetchAll() (not always desirable)
4. Use $stmt->closeCursor() after each $stmt->fetch() (this mostly worked, however, I still had several cases where it didn't)
5. Change PHP MySQL library from php-mysql to php-mysqlnd (probably what I will do if no better answer)

I have almost same problem. My first query after connection to db return empty result and drop this error. Enabling buffer doesn't help.

My connection code was:

try { 
    $DBH = new PDO("mysql:host=$hostname;dbname=$db_name", $username, $password, 
    array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET CHARACTER SET utf8; SET NAMES utf8",
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_NUM));
} 
catch(PDOException $e) { echo $e->getMessage(); }

Solution in my way was to remove initial command:

PDO::MYSQL_ATTR_INIT_COMMAND => "SET CHARACTER SET utf8; SET NAMES utf8"

Here is a correct code:

try { 
    $DBH = new PDO("mysql:host=$hostname;dbname=$db_name", $username, $password, 
    array(PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_NUM));
} 
catch(PDOException $e) { echo $e->getMessage(); }

And MYSQL_ATTR_USE_BUFFERED_QUERY is not forced to true. It's set as default.