Odd though it may seem, this principle explains some of PHP's greatest security weaknesses. PHP does not explicitly use Secure By Design as a guiding principle when executing features. I'm sure its in the back of developers' minds just as I'm sure it has influenced many if their design decisions, however there are issues when you consider how PHP has influenced the security practices of PHP programmers. The result of not following Secure By Design is that all applications and libraries written in PHP can inherit a number of security vulnerabilities, hereafter referred to as "By-Default Vulnerabilities".
He focuses on what he sees as a responsibility of those creating the language to either default to a more secure architecture or provide information as to why their choices could cause problems. In the extended version of the post, he talks about some specific issues that the language has including SSL/TLS misconfiguration, openings for XML entity injection attacks and limited native filtering for cross-site scripting.