As a human, and a programmer, I find it remarkably easy to make mistakes, and overlook certain things, especially when pressed for time.
It's easy, and perhaps all too tempting, to blame a certain language, for being too accessible for its own good. But that would be glossing over the larger problem of human fallibility, regardless of the language chosen to program in.
Granted, we've come a long way since assembly language, and I think I would be far more productive programming in a more modern language, such as PHP, Python, Ruby, or Java.
PHP (and other scripting languages) have in fact lowered the barrier to entry. That may mean that more newcomers to programming try PHP first. But that certainly does not also mean that all PHP programmers are somehow less qualified, or less able to learn from their mistakes than programmers of other languages.
Rasmus Lerdorf created PHP in its original form back in 1994, it has evolved considerably since then. In its most modern incarnation, it supports object oriented programming, as well as superb frameworks, such as Symfony. PHP as a language has broken free from its original constraints, and has grown to offer great flexibility in how programmers can choose to use it. You can use it to create a 9,000 line script of spaghetti code, or you can use it within the context of a modern, MVC framework, such as Symfony: it's your choice!
I strongly suspect that security vulnerabilities are not restricted to a single language.
It's tempting to write off all PHP programmers as somehow less capable, or more prone to writing insecure code. But I wonder how much of that is language bias, and how much of it is fact?