The vulnerability stems from insecure template markup in the Boost theme's secure layout (navbar-secure.mustache), not from specific PHP functions. The commit diff shows removal of an <a> tag linking to config.wwwroot in the template file, which allowed navigation to the home page. While template rendering mechanisms are involved, no specific PHP functions were identified as vulnerable - the issue resides purely in the template structure allowing unintended navigation. The added test files (securelayout.feature, securetestpage.php) validate the fix but don't represent vulnerable code.