MySQLi query results: Best approach, do you close, free, both?

Question

I have some questions about using MySQLi, queries, and related memory management. The code here is just to clarify my questions, so don\'t dump on it for error checking, etc. I

Answer 1

As rare as they are, in my opinion memory leaks are a nightmare to find and correct. I go out of my way to avoid them. Below is the pattern I use, based on the code you supplied:

$db = NULL;
try {
    $dbPool = "p:$dbhost"; // question 3: use pooling
    $db = new mysqli($dbPool, $un, $ps, $dbname);
    if ($db->connect_errno) {
        throw new Exception('' . $db->connect_error . ' ' . $db->connect_errno 
                . "\n" . $un . '@' . $dbhost . ' ' . $dbname);
        // NOTE: It's commonly considered a security 
        // risk to output connection information e.g.
        // host, user and database names.
    }

    $query = "SELECT field1, field2 ".
             "FROM table1 ".
             "WHERE field1={$some_value}";

    $results = NULL;
    try {

        if (!$results = $db->query($query)) {
            throw new Exception($db->error . " " . $db->errno 
                    . "\n" . $query);
            // NOTE: It's commonly considered a security 
            // risk to output SQL ($query).
        }
        while ($result = $results->fetch_object()) {
            // Do something with the results
        }

    } catch (Exception $ex) {
        // log, report, or otherwise handle the error
    }
    if ($results) {
        $results->free(); // question 1: why risk it?
    }

    $query = "SELECT field1, field2 ".
             "FROM table2 ".
             "WHERE field1={$some_value2}";

    $results = NULL; 
    try {

        if (!$results = $db->query($query)) {
            throw new Exception($db->error . " " . $db->errno 
                    . "\n" . $query);
            // NOTE: It's commonly considered a security 
            // risk to output SQL ($query).
        }            
        while ($result = $results->fetch_object()) {
            // Do something with the second set of results
        }

    } catch (Exception $ex) {
        // log, report, or otherwise handle the error
    }
    if ($results) {
        $results->free(); // question 2: again, why risk it?
    }

} catch (Exception $ex) {
    // log, report, or otherwise handle the error
}
if ($db) {
    $db->close();
}

In my opinion, connection pooling increases the chances for a memory leak, but according to the manual, the connection pooling libraries do a lot of cleanup for you automatically:

The persistent connection of the mysqli extension however provides built-in cleanup handling code. The cleanup carried out by mysqli includes:

Rollback active transactions

Close and drop temporary tables

Unlock tables

Reset session variables

Close prepared statements (always happens with PHP)

Close handler

Release locks acquired with GET_LOCK()

This ensures that persistent connections are in a clean state on return from the connection pool, before the client process uses them.

source: http://php.net/manual/en/mysqli.persistconns.php

I also agree with Pascal MARTIN that it's a good idea to open your connection at the beginning of your script and close it at the end. I think connection pooling makes that less important, but still a good idea.