From SQLite's point of view, this looks to be okay.

Here's a snippet from the SQLite docs[1], in the "Opening
A New Database Connection" section[2]:

  If the filename is an empty string, then a private,
  temporary on-disk database will be created. This 
  private database will be automatically deleted as 
  soon as the database connection is closed.

[1] http://www.sqlite.org/c3ref/intro.html
[2] http://www.sqlite.org/c3ref/open.html

Is this temp database being created in /tmp or in the current directory?

Also, isn't this basically the same thing as using :memory to create a temp db, but on disk? I would have thought there would be a :disk option instead of assuming the user wanted to create a temp db on disk, and had not made a coding error.

Is there, or should there be a note about this behavior in the PHP docs? I know I was stumped by my try/catch not working as expected to warn me.

Switching to documentation problem. Maybe it would be worth to add a note?

The behavioral change has been introduced as fix for bug
#52010[1]. How temporary databases are handled by Sqlite3 is
#detailed in [2].

[1] <http://svn.php.net/viewvc/php/php-src/trunk/ext/pdo_sqlite/sqlite_driver.c?r1=300612&r2=300611&pathrev=300612>
[2] <https://www.sqlite.org/inmemorydb.html#temp_db>