Server-side programming is one of the key technologies that support today's WWW environment. It makes it possible to generate Web pages dynamically according to a user's request and to customize pages for each user. However, the flexibility obtained by server-side programming makes it much harder to guarantee validity and security of dynamically generated pages.To check statically the properties of Web pages generated dynamically by a server-side program, we develop a static program analysis that approximates the string output of a program with a context-free grammar. The approximation obtained by the analyzer can be used to check various properties of a server-side program and the pages it generates.To demonstrate the effectiveness of the analysis, we have implemented a string analyzer for the server-side scripting language PHP. The analyzer is successfully applied to publicly available PHP programs to detect cross-site scripting vulnerabilities and to validate pages they generate dynamically.