PHP, PDO, SQLite, mysterious lock problem

Let's start with the conclusion:
If doing a prepare() or query() with PDO and sqlite, and then you want to do something else with sqlite in the same PHP function then unset the first PDOStatement, before trying to do that something else.

As an example here is my code, to get a unique ID ($dbh is a PDO connection to an sqlite):

function get_next_id($dbh){
$q='SELECT next FROM MyNextId';
$obj=$dbh->query($q); //Throws on error
$d=$obj->fetch(PDO::FETCH_NUM);
$next_id=$d[0];

$q='UPDATE MyNextId SET next=next+1';
$row_count=$dbh->exec($q); //Throws on error
if($row_count==0)throw new Exception("Failed to execute ($q)\n");

return $next_id;
}

It works on Ubuntu 10.04, with sqlite 3.6.22, but fails on Centos 5.6, with sqlite 3.3.6, with this message:
exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 6 database table is locked'

I went through the whole changelog from 3.3.6 to 3.6.22, but got no clues (though I am now impressed with how active and organized the sqlite development is). But finally I tracked down this article on someone getting similar errors.

And that was it. I could have used $obj->closeCursor(), but deleting $obj is just as good:

function get_next_id($dbh){
$q='SELECT next FROM MyNextId';
$obj=$dbh->query($q); //Throws on error
$d=$obj->fetch(PDO::FETCH_NUM);
$next_id=$d[0];

unset($obj);

$q='UPDATE MyNextId SET next=next+1';
$row_count=$dbh->exec($q); //Throws on error
if($row_count==0)throw new Exception("Failed to execute ($q)\n");

return $next_id;
}

If you are doing just one PDO action per function then there is no need, because exiting the function will automatically do the unset.

(I don't know why this is a problem on sqlite 3.3.6 but not sqlite 3.6.22... in fact, I suspect it may be due a difference in the PDO or PHP version or configuration instead. Apologies for the loose end!)