php and =&

I'm looking through various documentation on MDB2 In some examples the assignment operator looks like it is mixed with a bitwise or logic operator - IE $mdb2 =& MDB2::factory($dsn); Other examples though just have $mdb2 = MDB2::factory($dsn); I'm trying to figure out exactly what =& does, why people would use it, and when it is appropriate to use, but I haven't found anything and I think my google skills need some practice as I don't know how to search for =& in google. In the php manual on operators I did fine &= discussed but that's the flip and it's use makes no sense in the context of setting a database handle. Any tips would be appreciated.