Description:
------------
---
From manual page: http://www.php.net/mongocollection.ensureindex#refsect1-mongocollection.ensureindex-parameters
---

db->designComment->ensureIndex('reply.id', array('unique'=>1, 'sparse'=>1))

Test script:
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db->designComment->ensureIndex('reply.id', array('unique'=>1, 'sparse'=>1))

Expected result:
----------------
When more the one reply.ids are null. Using option array('unique'=>1) can not successfully ensureIndex.

Actually, the mongo offer the 'sparse' option to work for this situation(http://docs.mongodb.org/manual/core/indexes/#synopsis)

Cited from the above like "You can combine the sparse index option with the unique indexes option so that mongod will reject documents that have duplicate values for a field, but that ignore documents that do not have the key."

And this option is already available in php, but the documentation does not list this option


Actual result:
--------------
{"v":1,
"key":{"reply.id":1},
"unique":true,
"ns":"XXXXX.designComment",
"sparse":1,
"name":"reply_id_1"}

The above proves the 'sparse' option is available in php