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How to get filters for input streams using stream_get_filters in PHP?

gitbox 2025-05-27

In PHP, stream_get_filters() is a very useful function that allows you to view currently available stream filters. A stream filter can be used to convert or process data when it is read or written, such as compression, encryption, encoding, etc. This article will explain how to use stream_get_filters() to view these filters and demonstrate a simple application example.

1. What is a flow filter?

The stream filter is part of the PHP stream wrapper, which is used to dynamically process data during streaming data transmission. For example, you can compress data before writing it to a file, or automatically decompress it when reading data from a file.

Common filters include:

  • string.rot13 : ROT13 encoding the string.

  • string.toupper : Converts a string to uppercase.

  • string.tolower : Converts a string to lowercase.

  • convert.base64-encode and convert.base64-decode : Base64 encoding or decoding the data.

2. Use stream_get_filters() to view available filters

You can use the stream_get_filters() function to get an array that lists all available stream filters in the current PHP environment:

 <?php
$filters = stream_get_filters();
echo "Available flow filters：\n";
print_r($filters);
?>

After running this code, you will get an output like this:

 Available flow filters：
Array
(
    [0] => zlib.*
    [1] => string.rot13
    [2] => string.toupper
    [3] => string.tolower
    [4] => convert.*
)

Note: Different PHP installations may list different filters depending on the extension module installed.

3. Practical application: Use filters for file writing

Let's demonstrate an example of using filters to convert file content to uppercase.

 <?php
$filename = 'output.txt';
$fp = fopen($filename, 'w');

// Apply when writing string.toupper Filter
stream_filter_append($fp, 'string.toupper');

fwrite($fp, "hello, gitbox.net!\n");
fwrite($fp, "this is a test.\n");

fclose($fp);

echo "Write complete，Please check the file $filename。\n";
?>

This code will create an output.txt file, which will automatically convert all letters to uppercase when written. After you open the file, the content will be:

 HELLO, GITBOX.NET!
THIS IS A TEST.

4. Use URL streams and filters to combine

PHP can not only operate local file streams, but also use fopen() to open URL streams (if allow_url_fopen is allowed). For example:

 <?php
$url = 'https://gitbox.net/data.txt';
$fp = fopen($url, 'r');

// Apply when reading ROT13 Filter
stream_filter_append($fp, 'string.rot13');

while (!feof($fp)) {
    echo fgets($fp);
}

fclose($fp);
?>

Assuming that the content on https://gitbox.net/data.txt is normal text, this code will encode the content in ROT13 and output it in real time.

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