Description:
------------
It appears that passing a large amount of data (possibly including heavily nested objects) through unserialize() causes a memory leak.

I've run into this problem using a SimpleSAMLphp script to keep memcache instances in sync (https://code.google.com/p/simplesamlphp/source/browse/trunk/bin/memcacheSync.php) when syncronizing large caches. The code makes repeated calls to unserialize() with SimpleSAML_Session objects and appears to leak memory within that call.

I've noticed that the refcount of the returned objects is 3 but I don't know if that's relevant.

Cutting out the call to unserialize() and, instead, just returning the string does not leak the memory. Returning the unserialized object directly after the unserialize() (rather than letting the rest of the function continue) exhibits the leak.

The test script doesn't leak anywhere near the amounts I've seen live data leaking (see http://pastebin.com/a15gbNek).

Test script:
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http://pastebin.com/0cSwhznQ


Expected result:
----------------
$ php test.php
Populating object...
Serializing...
Looping...
19000: 7.55Mb
18000: 7.55Mb
17000: 7.55Mb
16000: 7.55Mb
15000: 7.55Mb
14000: 7.55Mb
13000: 7.55Mb
12000: 7.55Mb
11000: 7.55Mb
10000: 7.55Mb
9000: 7.55Mb
8000: 7.55Mb
7000: 7.55Mb
6000: 7.55Mb
5000: 7.55Mb
4000: 7.55Mb
3000: 7.55Mb
2000: 7.55Mb
1000: 7.55Mb
0: 7.55Mb

Actual result:
--------------
$ php test.php
Populating object...
Serializing...
Looping...
19000: 7.55Mb
18000: 7.57Mb
17000: 7.59Mb
16000: 7.60Mb
15000: 7.60Mb
14000: 7.61Mb
13000: 7.61Mb
12000: 7.62Mb
11000: 7.62Mb
10000: 7.62Mb
9000: 7.62Mb
8000: 7.62Mb
7000: 7.62Mb
6000: 7.62Mb
5000: 7.62Mb
4000: 7.62Mb
3000: 7.62Mb
2000: 7.62Mb
1000: 7.62Mb
0: 7.62Mb