Entry for CVE-2008-3659 says:

Buffer overflow in the memnstr function in PHP 4.4.x before 4.4.9 and PHP 5.x through 5.2.6 allows context-dependent attackers to cause a denial of service (crash) and possibly execute arbitrary code via the delimiter argument to the explode function.

NOTE: the scope of this issue is limited since most applications would not use an attacker-controlled delimiter, but local attacks against safe_mode are feasible.