Pdo 3, multiple runnings: error: 17 database schema has changed - php

Question

I have this test code:
session_start();
session_write_close();
set_time_limit(0);
for ($i = 1; $i < 1000; $i++)
{
if (rand(1,5) == 1)
{
$file_db = new PDO('sqlite:x.db');
$file_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$file_db->exec('DROP TABLE IF EXISTS messages');
$file_db = null;
}
$file_db = new PDO('sqlite:x.db');
$file_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$file_db->exec('CREATE TABLE IF NOT EXISTS messages (id INTEGER PRIMARY KEY AUTOINCREMENT,title TEXT,message TEXT,time INTEGER)');
$file_db->query('INSERT INTO messages VALUES (null, "'.rand(1,9999).'", "'.rand(1,99).'", date("now"))');
$file_db = null;
}
$file_db = new PDO('sqlite:x.db');
$file_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$i = 0;
foreach ($file_db->query('SELECT * FROM messages') as $m)
{
$i++;
echo $i.' -> '.$m['id'].'<br>';
}
with this code Im testing if the PDO 3 works with multiple concurrencies. If this code runs in one thread, all is ok. But as soon as I start two instance of this, I get this message:
Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 17 database schema has changed' in code.php:* Stack trace: #0 code.php(*): PDO->query('INSERT INTO mes...') #1 {main} thrown in code.php on line *
the error line is where the INSERT INTO is. Why does this happen and how to dodge it? (Im creating a cache-system where there is do a possibility that tables gets created and dropped. I know there is TRUNCATE, I just want to know why this happens)

Answer 1

[...] as soon as I start two instance of this, I get this message: [...] SQLSTATE[HY000]: General error: 17 database schema has changed [...] Why does this happen and how to dodge it?
Sqlite is a single file database. You open it in parallel and you then change the database structure. Inserting data while you just dropped the table to insert into just does not work because the table is gone. This explains why it happens.
As you hopefully now have understood that inserting into a non-existing table is not possible, the solution might be clearly visible: Only insert into an existing table.
To ensure that you create a table after you've dropped it, it might be helpful to do such operations in transactions: https://www.sqlite.org/lang_transaction.html

Answer 2

Try$file_db = new PDO(realpath('sqlite:x.db'));
for
$file_db = new PDO('sqlite:x.db');