### I. Web Server Security
PHP is essentially just a module feature of the Web server, so the first step is to ensure the security of the Web server. Of course, for the Web server to be secure, the system itself must also be secure, which leads to an endless chain of dependencies. PHP can integrate with various Web servers, but here we will only discuss Apache. It is highly recommended to install and start Apache in chroot mode. This way, even if there are vulnerabilities in Apache, PHP, or their scripts, only the confined system will be affected, leaving the actual system safe from harm.
However, using Apache in chroot mode can bring some inconvenience to applications. For example, when connecting to MySQL, you must use the TCP connection via the 127.0.0.1 address instead of using localhost for socket connections, which may slightly reduce efficiency. Additionally, sending emails using the mail function can also become problematic because php...