Odd behavior with SQLite3 non-SELECT prepared transactions and fetchArray() - php

Question

I ran into an odd issue earlier today which had me stumped for a while: I was doing precisely one transaction (confirmed umpteen different ways), and yet I was adding two elements to my SQLite3 database.
Finally twigged as to the possible cause, and tracked it down.
Here's a very minimal testcase which I've written to be as simple and straightforward as possible:
<?php
function dostuff($f) {
#unlink($f);
$db = new SQLite3($f);
$db->exec('CREATE TABLE test (a TEXT)');
$s = $db->prepare('INSERT INTO test (a) VALUES (:a)');
$s->bindValue(':a', 'OHI');
return $s->execute();
}
$f = '__test1__.sqlite3';
dostuff($f);
system('sqlite3 '.$f.' .dump');
print "\n---------------------\n\n";
$f = '__test2__.sqlite3';
$r = dostuff($f);
while (($row = #$r->fetchArray(SQLITE3_ASSOC)) !== false) {
print "LOOP EXECUTED!\n";
}
system('sqlite3 '.$f.' .dump');
?>
Firstly, dostuff creates a new database, which is removed if it exists. Then I create a test table, set up a prepared transaction, run the prepared transaction, and then the function returns the SQLite3Result object from $s->execute().
(I made a function for this code because it gets run twice, to keep the SLOC down.)
Here's what I understand is going on:
I run dostuff() with a first test file.
I run dostuff() with a second test file, but then I grab the SQLite3Result that the function returns, and then iterate over that as though I've just run a SELECT statement and I need to grab the results.
In both cases I dump the resulting database that's been created using system(sqlite3 .dump), as this is a simple, effective way to dump DBs with well-known, predictable results, and I wanted to avoid writing my own potentially buggy code.
This is the output I get on my machine, with PHP 7.0.5 and SQLite3 3.12.0 (PHP module version 0.7):
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE test (a TEXT);
INSERT INTO "test" VALUES('OHI');
COMMIT;
---------------------
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE test (a TEXT);
INSERT INTO "test" VALUES('OHI');
INSERT INTO "test" VALUES('OHI');
COMMIT;
Note that my code contains a loud "LOOP EXECUTED" line inside the fetchArray() loop, but this isn't showing up. The only output that is being presented is the output of the system() calls and the print statement displaying the dashes.
I crashed into this as I was building a DB helper class that would run statements for me and handle retrieving the results (my queries return very very small result sets so gathering all of them simplifies everything with no cost).
I've now written prepared-statement query() and exec() functions that do and do not try to fetch a result set, respectively - now I just have to remember to use the right ones in the right places!
TL;DR: INSERT + fetchArray() runs the query twice.
My question: Is this a bug? Or is it known behavior in SQLite3?