Stream Filtering with SplFileObject

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Since PHP4 days, the language has been able to interact with streams, a resource object that can be read from or written to in a linear fashion. This feature can help us applying filters function on streams while reading from or written to them. This can be interesting if you want, for instance, to compress or decompress, on the fly, a file before or after accessing it.
To apply those filters, the developer must use PHP stream filtering functions stream_filter_* and/or the php meta wrapper php://filter and have access to a PHP file resource. For instance, any resource returned by fopen can be filtered using PHP.

$path = 'path/to/my/file.txt';
$fp = fopen($path, 'r');
$filter1 = stream_filter_append($fp, 'string.toupper');
$filter2 = stream_filter_prepend($fp, 'string.rot13', STREAM_FILTER_READ);
$res = fgets($fp); //the output text is ROT13 and then uppercased
$res = fgets($fp); //the output text is only uppercased

The stream filters can be applied when reading,  writing or on both actions. They can be removed at any given time. This feature is truly powerful.

Now let’s jump to today’s post-PHP 5.3+ world where objects like SplFileObject where created to offer an OOP approach to file interaction. With the added benefit of using Iterator and many other goodies, being able to apply PHP stream filtering features on a SplFileObject as easily as on a file resource would be great but that is not currently possible. The main reason being that the SplFileObject class does not expose it’s internal file pointer resource, so using any stream_filter_* is simply not possible adding stream filters to an instantiate SplFileObject object is not possible.

If you want to apply stream filters to a SplFileObject object, you are required to instantiate the object using the meta wrapper php://filter.

$path = 'path/to/my/file.txt';
$file = new SplFileObject('php://filter/read=string.rot13|string.toupper/resource='.$path, 'r');
$file->fgets();  //the output text is ROT13 and then uppercased

This technique works but has several limitations, here are two that come directly in mind:

  • Writing the PHP meta-wrapper can easily become a headache, if you apply several filters.
  • the meta wrapper restricts the use of stream filters, as you can not append/prepend the filters after the object instantiation!!
  • you can not set the stream filter mode (read and/or write) on a filter base

I understand that the stream_filter_* functions were first design to tackle those limitations when using a file resource. It’s a bit strange that the same problem was re-introduce when SplFileObject was designed.

To solve theses issues PHP could expose the internal file pointer resource used by SplFileObject, there’s a php bug entry for that. But I can imagine that exposing the file pointer resource would have some nasty consequences when using the SplFileObject. For instance someone could use fclose directly on the resource and they’re maybe worse case scenario.

Another solution would be to implement stream_filter_append and stream_filter_prepend as new methods for the SplFileObject.

$file = new SplFileObject('/path/to/my/file', 'r');
$filter1 = $file->appendStreamFilter('string.toupper');
$filter2 = $file->prependStreamFilter('string.rot13', STREAM_FILTER_READ);

This proposal would be safer to implement, IMHO.

Now don’t get me wrong, this is not another attempt to go OOP with everything that exists in PHP, but I think I would benefit any codebase that relies heavily on SplFileObject, like my PHP CSV manipulation library League\Csv.

Update 2014-04-29: After thinking over this solution a bit, I think there’s another solution. I think we could implement an new PHP interface called Streamable and SplFileObject would implement this interface. This interface would act like the Traversable interface meaning that it can not be access directly in userland but it would allow classes implementing it to work with functions like stream_filter_append and stream_filter_prepend. These functions would accept a file resource pointer and/or a Streamable class. This interface could be use on any PHP function apart from file specific function to allow SplFileObject to work on many other situations. It would expose internally only the object file pointer resource to the native functions.

What do you think ?

PS: I don’t know the amount of code changes, it would require in the PHP source code to implement any of those proposal. But I think something must be done to allow a better use of stream filters with SplFileObject.

2 thoughts on “Stream Filtering with SplFileObject

  1. I totally agree, man. I really like your last idea about the Streamable interface. I’ve been running into the same issues (I’m writing my own CSV library much like your own). I actually made the decision to just scrap SplFileObject altogether because it was too inflexible and I couldn’t integrate it with the rest of my IO/stream classes. Something like you suggested would make SplFileObject usable for me. It would make a huge difference. Thanks for the great read and for the inspiration (your CSV library). Keep up the good work!

  2. By the way, your use of the php://filter meta wrapper to apply filters to a filename before passing it to SplFileObject, was brilliant! I hadn’t even considered doing that! Granted, it has its disadvantages, but it’s still very clever. I’ll definitely be looking for an excuse to use it now 😉

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