If you are attempting to use numeric field selection with MongoCollection::find() and run into the error:
PHP Fatal error: Uncaught exception 'MongoException' with message 'field names must be strings'
It is caused by PHP converting an array key that is defined as a string into an integer, so:
array('123' => 0);
becomes:
array(123 => 0);
Unfortunately mongo requires strings for all field names. A quick workaround (found after browsing code, not yet documented) to the problem is to use an object instead of an array:
$fields = new stdClass;
$fields->{123} = 0;
Problem solved.
