A filesystem stores a lot of additional information about files aside from their actual contents. This information includes such particulars as the file size, directory, and access permissions. If you're working with files, you may also need to manipulate this metadata. PHP gives you a variety of functions to read and manipulate directories, directory entries, and file attributes. Like other file-related parts of PHP, the functions are similar to the C functions that accomplish the same tasks, with some simplifications.
Files are organized with inodes. Each file (and other parts of the filesystem, such as directories, devices, and links) has its own inode. That inode contains a pointer to where the file's data blocks are as well as all the metadata about the file. The data blocks for a directory hold the names of the files in that directory and the inode of each file.
PHP provides a few ways to look in a directory to see what files it holds. The DirectoryIterator class (available in PHP 5 and later) provides a comprehensive object-oriented interface for directory traversal. Example 24-1 uses DirectoryIterator to print out the name of each file in a directory.
The opendir( ), readdir( ), and closedir( ) functions offer a procedural approach to the same task, as demonstrated in Example 24-2. Use opendir( ) to get to get a directory handle, readdir( ) to iterate through the files, and closedir( ) to close the directory handle. 
Procedural directory iteration
In this chapter, we generally use DirectoryIterator for examples.
The filesystem holds more than just files and directories. On Unix, it can also hold symbolic links. These are special files whose contents are a pointer to another file. You can delete the link without affecting the file it points to. To create a symbolic link, use symlink( ), as in Example 24-3.
Making a symbolic link
The code in Example 24-3 creates a symbolic link called images in /www/docroot that points to /usr/local/images.
To find information about a file, directory, or link you must examine its inode. The function stat( ) retrieves the metadata in an inode for you. Recipe 24.2 discusses stat( ). PHP also has many functions that use stat( ) internally to give you a specific piece of information about a file. These are listed in Table 24-1.
On Unix, the file permissions indicate what operations the file's owner, users in the file's group, and all users can perform on the file. The operations are reading, writing, and executing. For programs, executing means the ability to run the program; for directories, executing is the ability to search through the directory and see the files in it.
Unix permissions can also contain a setuid bit, a setgid bit, and a sticky bit. The setuid bit means that when a program is run, it runs with the user ID of its owner. The setgid bit means that a program runs with the group ID of its group. For a directory, the setgid bit means that new files in the directory are created by default in the same group as the directory. The sticky bit is useful for directories in which people share files because it prevents nonsuperusers with write permission in a directory from deleting files in that directory unless they own the file or the directory.
When setting permissions with chmod( ) (see Recipe 24.3), permissions must be expressed as an octal number. This number has four digits. The first digit is any special setting for the file (such as setuid or setgid). The second digit is the user permissions'what the file's owner can do. The third digit is the group permissions'what users in the file's group can do. The fourth digit is the world permissions'what all other users can do. To compute the appropriate value for each digit, add together the permissions you want for that digit using the values in Table 24-2. For example, a permission value of 0644 means that there are no special settings (the 0), the file's owner can read and write the file (the 6, which is 4 (read) + 2 (write)), users in the file's group can read the file (the first 4), and all other users can also read the file (the second 4). A permission value of 4644 is the same, except that the file is also setuid.
The permissions of newly created files and directories are affected by a setting called the umask, which is a permission value that is removed or masked out from the initial permissions of a file (0666) or directory (0777). For example, if the umask is 0022, the default permissions for a new file created with touch( ) or fopen( ) are 0644 and the default permissions for a new directory created with mkdir( ) are 0755. You can get and set the umask with the function umask( ). It returns the current umask and, if an argument is supplied to it, changes the umask to the value of that argument. Example 24-4 shows how to make the permissions on newly created files prevent anyone but the file's owner (and the superuser) from accessing the file.
Changing the default file permissions
In Example 24-4, the first call to umask( ) masks out all permissions for group and world. After the file is created, the second call to umask( ) restores the umask to the previous setting. When PHP is run as a server module, it restores the umask to its default value at the end of each request. Windows has a different (and more powerful) system for organizing file permissions and ownership, so PHP's umask( ) function (like every other permissions-related function) isn't available on Windows.