The Linear Congruential Generator (LCG) in PHP before 5.2.13 does not provide the expected entropy, which makes it easier for context-dependent attackers to guess values that were intended to be unpredictable, as demonstrated by session cookies generated by using the uniqid function.
http://www.vupen.com/english/advisories/2010/3081
http://www.vupen.com/english/advisories/2010/0479
http://www.redhat.com/support/errata/RHSA-2010-0919.html
http://www.php.net/releases/5_2_13.php
http://www.php.net/ChangeLog-5.php