The stream extension of PHP is one of the batteries included in its packaging: a uniform interface to access many different types of data, relying on the common abstraction of a stream (not unlike, for example, Java's InputStream and OutputStream). It is not an object-oriented interface, but it is uniform across resource types, allowing reads and writes on files, processes, TCP sockets, and more; it's still polymorphism in action.
fopen() is more than you think
How do you read the contents of a file into a PHP variable? Sure, there are many shortcuts if you just want the whole file to be put into memory, such as file_get_contents(). However, assuming you want to lower your memory footprint and deal with arbitrarily large file, you can create a file descriptor and stream data in:
$fp = fopen("file.txt", "r"); $line = fgets($fp);
PHP interface in this case is similar to C's one. fgets() reads a line from a text file, but you have access to different methods, like fread() who reads an amount of bytes instead.
PHP extends the syntax of fopen() argument with a series of wrappers, that allow not only to access the filesystem but also other types of resources; for example, web pages:
$fp = fopen("http://www.example.com/file.txt", "r"); $line = fgets($fp);
Specialized functions that create streams
fopen() special syntax covers many cases, but sometimes creating a stream with specialized functions is simpler and clear, like in the case of TCP sockets:
$socket = fsockopen('example.com', 25); $line = fgets($socket);
This is even more clear in the case of external processes, which may need many options to be specified such as environment variables and arguments. In this case, the created streams represent the standard input, standard output and standard error of the child process.
$process = proc_open('/bin/command', array(0 => 'r', 1 => 'w', 2 => 'w'), $pipes); $line = fgets($pipes);
However, why limit yourself to local processes? You may want to execute them on a remote machine via ssh (we're back to fopen()):
$fp = fopen('ssh2.exec://user:email@example.com:22/usr/local/bin/somecmd'); $line = fgets($fp);
Or you can read from standard input:
$fp = fopen('php://stdin', 'r'); $line = fgets($fp);
Or the body of a PUT or POST request, since you don't want to keep that 50 MB zip archive into memory.
$fp = fopen('php://input', 'r'); $line = fgets($fp);
Or accessing a file insize that zip archive without expanding it:
$fp = fopen('zip://./foo.zip#bar.txt', 'r'); $line = fgets($fp);
Simulate a data stream
I learned from a colleague of mine a special kind of data stream to instantiate for testing, the data://* one; it allows you to specify a literal input in base64 format. With this stream you can avoid creating temporary files inside automated tests, which require bookkeeping and tearDown() methods:
// prints "I love PHP" echo file_get_contents('data://text/plain;base64,SSBsb3ZlIFBIUAo=');
What works on file descriptors?
So we now know the type of resources that streams support: files, sockets, processes, and many more. But which functions can we run on the $fp file descriptors?
First of all, the siblings of fopen() are available for many operations. We have seen fgets() in action and we can add fread() and fwrite() to that; even fseek() works on many streams, sometimes only seeking forward in the contents.
Moreover, the stream extension provides a bunch of functions that work on any descriptor. You should only exclude from this list the stream_socket_*() which are specific to the management of TCP and UDP (client and server) sockets.
Not all of the operations are available on all streams, due to their constraints: for example, some allow only reads, not writing as is the case for HTTP; in our example, fopen()ing URLs works with GET requests and can only read the content of a resource, why you should use curl and other tools to make complex requests.
Still, even with these incompatibilities between streams, you only have to learn a single API to work on any kind of resource, along with the syntax to create that kind of stream. You can even create your own, userland stream wrapper in case it's not shipped with PHP.