PHP stream_copy_to_stream() Function: Usage and Examples

In PHP, stream_copy_to_stream() is useful when your application needs a specific operation rather than manually reproducing it. Copies data from one open stream to another, optionally limited by length and offset.

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

What Is stream_copy_to_stream() in PHP?

Copies data from one open stream to another, optionally limited by length and offset.

Signature:

stream_copy_to_stream($from, $to, ?int $length = NULL, int $offset = 0): int|false

Parameters:

$from — Readable source stream resource.
$to — Writable destination stream resource.
$length — Requested byte count or upper bound; reads can return fewer bytes.
$offset — Starting position or byte offset; interpret relative to the function.

Return value: int|false.

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

$stream = fopen("php://temp", "w+");
fwrite($stream, "alpha\nbeta\n");
rewind($stream);
$source = $stream;
$destination = fopen("php://temp", "w+");
$copiedBytes = stream_copy_to_stream($source, $destination);
fclose($stream);

The important operation is stream_copy_to_stream(). Copies data from one open stream to another, optionally limited by length and offset.

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

$stream = fopen("php://temp", "w+");
fwrite($stream, "gamma\ndelta\n");
rewind($stream);
$source = $stream;
$destination = fopen("php://temp", "w+");
$copiedBytes = stream_copy_to_stream($source, $destination);
fclose($stream);

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

$stream = fopen("php://temp", "w+");
fwrite($stream, "alpha\nbeta\n");
rewind($stream);
$source = $stream;
$destination = fopen("php://temp", "w+");
$copiedBytes = stream_copy_to_stream($source, $destination);
// Confirm the return value before using the result.
fclose($stream);

Practical note: Copies data from one open stream to another, optionally limited by length and offset.

Important Behavior and Edge Cases

Copies data from one open stream to another, optionally limited by length and offset.
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_copy_to_stream()`	moves bytes directly between two streams.
`stream_get_contents()`	reads the bytes into a PHP string.

Common Beginner Mistakes

Mistake	Correct Approach
Assuming `stream_copy_to_stream()` can be used without its required setup	Provide the correct file, stream, state, key or object before calling it; Copies data from one open stream to another, optionally limited by length and offset.
Assuming a resource result is always successful	Check false returns, actual byte counts, and resource state before using the result.
Confusing local and remote stream behavior	Read the wrapper capabilities and handle unsupported operations explicitly.

Key Takeaways

stream_copy_to_stream() copies data from one open stream to another, optionally limited by length and offset.
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: Copies data from one open stream to another, optionally limited by length and offset. Build in error handling and use this function where its specific behavior matches the application requirements.