(PECL mongo >=0.9.0)
Creates an index on the specified field(s) if it does not already exist.
public bool MongoCollection::ensureIndex ( string|array $key|keys [, array $options = array() ] )
Creates an index on the specified field(s) if it does not already exist. Fields may be indexed with a direction (e.g. ascending or descending) or a special type (e.g. text, geospatial, hashed).
Note:
This method will use the