Web application security has emerged as one of the most persistent and complex challenges in modern software engineering, driven by the rapid evolution of programming languages, frameworks, and deployment environments. Despite decades of research and practical countermeasures, vulnerabilities such as cross-site scripting, authentication flaws, and insecure session management continue to dominate real-world incident reports. This research article presents a comprehensive, language-centric investigation of web application security vulnerabilities, with particular emphasis on PHP and Java-based ecosystems, drawing strictly upon established empirical and conceptual studies in the literature. The study synthesizes findings from vulnerability field studies, empirical analyses of open-source software, framework-level security evaluations, and developer-centered security research. By examining vulnerabilities through the lenses of programming language design, framework abstraction, developer behavior, and performance–security trade-offs, this work offers a holistic understanding of why certain classes of vulnerabilities persist across technological generations. The methodology relies on qualitative comparative analysis of prior empirical investigations, complemented by theoretical reasoning rooted in software reliability, security engineering, and human factors. The results reveal that while modern frameworks introduce robust security mechanisms, they also introduce new forms of complexity that can obscure security assumptions and create configuration-dependent risks. The discussion highlights critical limitations in existing security models, including overreliance on framework defaults, insufficient developer security education, and the tension between performance optimization and defensive rigor. The article concludes by outlining future research directions focused on adaptive security policies, language-aware vulnerability prevention, and the integration of security education into the core of software development practice.