In memory sqlite has some limitations. The memory space could be the request, the session, but no way seems documented to share a base in memory among users.
For a request, open your base with the code
$pdo = new PDO( 'sqlite::memory:');
and your base will disapear on next request.
For session persistency
$pdo = new PDO(
array(PDO::ATTR_PERSISTENT => true)