Andrew van der Stock has written a strong criticism of PHP's insecurity. Andrew is a seasoned security expert and a major contributor to OWASP, and he states:
After writing PHP forum software for three years now, I've come to the conclusion that it is basically impossible for normal programmers to write secure PHP code. It takes far too much effort.
He continues, citing specific areas where he thinks PHP is weak and asserting that "PHP must now mature and take on a proper security architecture."
Many of the insecure features he cites (register_globals, magic_quotes_gpc, and safe_mode) are slated to be removed in PHP 6, but his core complaint seems to revolve around the fact that PHP makes too much power too easily accessible, often granting developers more power and flexibility than they realize (e.g., wrappers).
Aside from minor language features like taint mode, I don't see what other platforms offer to help prevent inexperienced developers from writing insecure code. Anyone care to enlighten me? :-)