PHP stream_filter_register() Function: Usage and Examples

In PHP, stream_filter_register() is useful when your application needs a specific operation rather than manually reproducing it. Registers a custom user-defined stream filter class under a filter name.

Stream functions work with PHP stream resources, wrappers, contexts or custom filters. Not every wrapper supports every operation.

What Is stream_filter_register() in PHP?

Registers a custom user-defined stream filter class under a filter name.

Signature:

stream_filter_register(string $filter_name, string $class): bool

Parameters:

$filter_name — Registered stream filter name.
$class — Class name used for returned file-information objects or stream wrappers.

Return value: bool.

Example 1: Basic Usage

This first example shows the method or function in a minimal context. For environment-dependent operations, adapt the required path, stream or connection before running it.

<?php

class UppercaseFilter extends php_user_filter {
    public function filter($input, $output, &$consumed, bool $closing): int {
        while ($bucket = stream_bucket_make_writeable($input)) {
            $consumed += $bucket->datalen;
            $bucket->data = strtoupper($bucket->data);
            stream_bucket_append($output, $bucket);
        }
        return PSFS_PASS_ON;
    }
}
stream_filter_register("demo.upper", UppercaseFilter::class);
$stream = fopen("php://temp", "w+");
stream_filter_append($stream, "demo.upper", STREAM_FILTER_WRITE);
fwrite($stream, "hello");
fclose($stream);

The important operation is stream_filter_register(). Registers a custom user-defined stream filter class under a filter name.

Example 2: Another Input or Scenario

The next example changes the input or surrounding context to show how the operation behaves in a second situation.

<?php

class UppercaseFilter extends php_user_filter {
    public function filter($input, $output, &$consumed, bool $closing): int {
        while ($bucket = stream_bucket_make_writeable($input)) {
            $consumed += $bucket->datalen;
            $bucket->data = strtoupper($bucket->data);
            stream_bucket_append($output, $bucket);
        }
        return PSFS_PASS_ON;
    }
}
var_dump(stream_filter_register("demo.upper", UppercaseFilter::class));

Compare the inputs and the handling of the return value with the first example. Avoid assuming that filesystem paths, resource states or binary bytes are interchangeable.

Practical Example: Applying the Operation

This example illustrates how the operation fits into a small application workflow. Paths, streams and secrets must be created or provided securely by the calling application.

<?php

class UppercaseFilter extends php_user_filter {
    public function filter($input, $output, &$consumed, bool $closing): int {
        while ($bucket = stream_bucket_make_writeable($input)) {
            $consumed += $bucket->datalen;
            $bucket->data = strtoupper($bucket->data);
            stream_bucket_append($output, $bucket);
        }
        return PSFS_PASS_ON;
    }
}
if (!in_array("demo.upper", stream_get_filters(), true)) { stream_filter_register("demo.upper", UppercaseFilter::class); }

Practical note: Registers a custom user-defined stream filter class under a filter name.

Important Behavior and Edge Cases

Registers a custom user-defined stream filter class under a filter name.
Check failure values and exceptions; do not assume every environment supports the same stream wrapper, extension or filesystem capability.
When handling paths, streams or external input, validate inputs before using results and release resources after use.

Comparison With Related Functions

Function or Method	Behavior
`stream_filter_register()`	registers a filter class by name.
`stream_filter_append()`	attaches an existing named filter to a stream.

Common Beginner Mistakes

Mistake	Correct Approach
Assuming `stream_filter_register()` can be used without its required setup	Provide the correct file, stream, state, key or object before calling it; Registers a custom user-defined stream filter class under a filter name.
Using bucket operations outside a filter callback	Stream bucket APIs are intended for php_user_filter subclasses and their provided brigades.
Forgetting consumed byte counts or filter order	Account for consumed data and test read/write filter ordering before deployment.

Key Takeaways

stream_filter_register() registers a custom user-defined stream filter class under a filter name.
Use appropriate input types and verify results before continuing.
Prefer specialized, safer APIs when the operation concerns untrusted input, encryption or network traffic.

In summary: Registers a custom user-defined stream filter class under a filter name. Build in error handling and use this function where its specific behavior matches the application requirements.