clue/stream-filter

A simple and modern approach to stream filtering in PHP

Table of contents

Why?
Usage
Install
License

Why?

PHP’s stream filtering system is great!

It offers very powerful stream filtering options and comes with a useful set of built-in filters. These filters can be used to easily and efficiently perform various transformations on-the-fly, such as:

read from a gzip’ed input file,
transcode from ISO-8859-1 (Latin1) to UTF-8,
write to a bzip output file
and much more.

But let’s face it: Its API is difficult to work with and its documentation is subpar. This combined means its powerful features are often neglected.

This project aims to make these features more accessible to a broader audience.

Lightweight, SOLID design - Provides a thin abstraction that is just good enough and does not get in your way. Custom filters require trivial effort.
Good test coverage - Comes with an automated tests suite and is regularly tested in the real world

Usage

This lightweight library consists only of a few simple functions. All functions reside under the Clue\StreamFilter namespace.

The below examples assume you use an import statement similar to this:

use Clue\StreamFilter as Filter;

Filter\append(…);

Alternatively, you can also refer to them with their fully-qualified name:

\Clue\StreamFilter\append(…);

append()

The append($stream, $callback, $read_write = STREAM_FILTER_ALL) function can be used to append a filter callback to the given stream.

Each stream can have a list of filters attached. This function appends a filter to the end of this list.

This function returns a filter resource which can be passed to remove(). If the given filter can not be added, it throws an Exception.

The $stream can be any valid stream resource, such as:

$stream = fopen('demo.txt', 'w+');

The $callback should be a valid callable function which accepts an individual chunk of data and should return the updated chunk:

$filter = Filter\append($stream, function ($chunk) {
    // will be called each time you read or write a $chunk to/from the stream
    return $chunk;
});

As such, you can also use native PHP functions or any other callable:

Filter\append($stream, 'strtoupper');

// will write "HELLO" to the underlying stream
fwrite($stream, 'hello');

If the $callback accepts invocation without parameters, then this signature will be invoked once ending (flushing) the filter:

Filter\append($stream, function ($chunk = null) {
    if ($chunk === null) {
        // will be called once ending the filter
        return 'end';
    }
    // will be called each time you read or write a $chunk to/from the stream
    return $chunk;
});

fclose($stream);

Note: Legacy PHP versions (PHP < 5.4) do not support passing additional data from the end signal handler if the stream is being closed.

If your callback throws an Exception, then the filter process will be aborted. In order to play nice with PHP’s stream handling, the Exception will be transformed to a PHP warning instead:

Filter\append($stream, function ($chunk) {
    throw new \RuntimeException('Unexpected chunk');
});

// raises an E_USER_WARNING with "Error invoking filter: Unexpected chunk"
fwrite($stream, 'hello');

The optional $read_write parameter can be used to only invoke the $callback when either writing to the stream or only when reading from the stream:

Filter\append($stream, function ($chunk) {
    // will be called each time you write to the stream
    return $chunk;
}, STREAM_FILTER_WRITE);

Filter\append($stream, function ($chunk) {
    // will be called each time you read from the stream
    return $chunk;
}, STREAM_FILTER_READ);

prepend()

The prepend($stream, $callback, $read_write = STREAM_FILTER_ALL) function can be used to prepend a filter callback to the given stream.

Each stream can have a list of filters attached. This function prepends a filter to the start of this list.

This function returns a filter resource which can be passed to remove(). If the given filter can not be added, it throws an Exception.

$filter = Filter\prepend($stream, function ($chunk) {
    // will be called each time you read or write a $chunk to/from the stream
    return $chunk;
});

fun()

The fun($filter, $parameters = null) function can be used to create a filter function which uses the given built-in $filter.

PHP comes with a useful set of built-in filters. Using fun() makes accessing these as easy as passing an input string to filter and getting the filtered output string.

$fun = Filter\fun('string.rot13');

assert('grfg' === $fun('test'));
assert('test' === $fun($fun('test'));

Please note that not all filter functions may be available depending on installed PHP extensions and the PHP version in use. In particular, HHVM may not offer the same filter functions or parameters as Zend PHP. Accessing an unknown filter function will result in a RuntimeException:

Filter\fun('unknown'); // throws RuntimeException

Some filters may accept or require additional filter parameters. The optional $parameters argument will be passed to the filter handler as-is. Please refer to the individual filter definition for more details. For example, the string.strip_tags filter can be invoked like this:

$fun = Filter\fun('string.strip_tags', '<a><b>');

$ret = $fun('<b>h<br>i</b>');
assert('<b>hi</b>' === $ret);

Under the hood, this function allocates a temporary memory stream, so it’s recommended to clean up the filter function after use. Also, some filter functions (in particular the zlib compression filters) may use internal buffers and may emit a final data chunk on close. The filter function can be closed by invoking without any arguments:

$fun = Filter\fun('zlib.deflate');

$ret = $fun('hello') . $fun('world') . $fun();
assert('helloworld' === gzinflate($ret));

The filter function must not be used anymore after it has been closed. Doing so will result in a RuntimeException:

$fun = Filter\fun('string.rot13');
$fun();

$fun('test'); // throws RuntimeException

Note: If you’re using the zlib compression filters, then you should be wary about engine inconsistencies between different PHP versions and HHVM. These inconsistencies exist in the underlying PHP engines and there’s little we can do about this in this library. Our test suite contains several test cases that exhibit these issues. If you feel some test case is missing or outdated, we’re happy to accept PRs! :)

remove()

The remove($filter) function can be used to remove a filter previously added via append() or prepend().

$filter = Filter\append($stream, function () {
    // …
});
Filter\remove($filter);

Install

The recommended way to install this library is through composer. New to composer?

$ composer require clue/stream-filter:~1.3

License

MIT

Monty Lee