Working with the File and Operating System with PHP

It's quite rare to write an application that is entirely self-sufficient--that is, a program that does not rely on at least some level of interaction with external resources, such as the underlying file and operating system, and even other programming languages. The reason for this is simple: As languages, file systems, and operating systems have matured, the opportunities for creating much more efficient, scalable, and timely applications have increased greatly as a result of the developer's ability to integrate the tried-and-true features of each component into a singular product. Of course, the trick is to choose a language that offers a convenient and efficient means for doing so. Fortunately, PHP satisfies both conditions quite nicely, offering the programmer a wonderful array of tools not only for handling file system input and output, but also for executing programs at the shell level. This chapter serves as an introduction to all such functionality, describing how to work with the following:

• Files and directories: You'll learn how to perform file system forensics, revealing details such as file and directory size and location, modification and access times, file pointers (both the hard and symbolic types), and more. 
   
• File ownership and permissions: All mainstream operating systems offer a means for securing system data through a permission system based on user and group ownership and rights. You'll learn how to both identify and manipulate these controls. 
   
• File I/O: You'll learn how to interact with data files, which will let you perform a variety of practical tasks, including creating, deleting, reading, and writing files. 
   
• Directory contents: You'll learn how to easily retrieve directory contents. 
   
• Shell commands: You can take advantage of operating system and other language-level functionality from within a PHP application through a number of built-in functions and mechanisms. You'll learn all about them. This chapter also demonstrates PHPs input sanitization capabilities, showing you how to prevent users from passing data that could potentially cause harm to your data and operating system.

Note  PHP is particularly adept at working with the underlying file system, so much so that it is gaining popularity as a command-line interpreter, a capability introduced in version 4.2.0. Although this topic is out of the scope of this book, you can find additional information in the PHP manual.