PHP のストリーム-PHPチュートリアル-php.cn

Streams in PHP

ローカルファイル、HTTP リクエスト、圧縮ファイルのいずれを扱う必要があったとしても、ストリームを扱ったことはありますが...本当に理解できましたか?

これは PHP で最も誤解されている概念の 1 つであると思います。その結果、基本的な知識が欠如しているためにかなりの数のバグが導入されるのを私は見てきました。

この記事では、ストリームとは実際何なのか、そしてストリームをどのように扱うのかについて説明していきます。ストリームを操作するために使用される多くの関数と多くの例を見ていきますが、それらすべてを何らかの形で「再文書化」するつもりはありません。

ストリームとは何かを学ぶ前に、まずリソースにアプローチする必要があります。

リソース

リソースは、ファイル、データベース、ネットワーク、SSH 接続などの外部リソースへの単なる参照またはポインタです。

リソースには、curl_init() によって作成されるcurl、proc_open および stream によって作成される process、fopen() や opendir などの関数によって作成されるなど、いくつかの種類があります。

ストリーム

ストリームは、PHP が共通の動作を持つリソースのタイプを一般化する方法です。つまり、リソースはカセットテープのように線形に読み書きできます (くそー、私も年をとりました)。ストリームの例としては、ファイルリソース、HTTP 応答本文、圧縮ファイルなどが挙げられます。

ストリームは、数バイトから数 GB のサイズのリソースを操作できるため、非常に便利です。たとえば、ストリームを完全に読み取ろうとすると、使用可能なメモリが使い果たされてしまいます。

fopen でストリームを作成する

 fopen(
    string $filename,
    string $mode,
    bool $use_include_path = false,
    ?resource $context = null
): resource|false

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fopen は、最初のパラメータに指定されたパスに応じて、ファイル または ネットワークリソース[1] を開きます。前に述べたように、このリソースのタイプはストリームです:

$fileStream = fopen('/tmp/test', 'w');
echo get_resource_type($fileStream); // 'stream'

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$filename がスキーム://の形式で指定された場合、それは URL であるとみなされ、PHP はローカルを処理するために、パスに一致するサポートされているプロトコルハンドラー/ラッパー (file:// など) を見つけようとします。ファイル、http:// - リモート HTTP/S リソースで作業するため、ssh2:// - SSH 接続を処理するため、または php:// - php://stdin などの PHP 独自の入出力ストリームへのアクセスを可能にします。 php://stdout および php://stderr.

$mode は、ストリームに必要なアクセスのタイプを定義します。つまり、読み取りアクセスのみが必要か、書き込みのみが必要か、読み取りと書き込みが必要か、ストリームの先頭または最後からの読み取り/書き込みが必要かどうかなどです。

モードは、作業しているリソースのタイプによっても異なります。例:

$fileStream = fopen('/tmp/test', 'w');
$networkStream = fopen('https://google.com', 'r');

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たとえば、ラッパー https:// を使用して書き込み可能なストリームを開くと、機能しません:

fopen('https://google.com', 'w'); // Failed to open stream: HTTP wrapper does not support writeable connections

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[1] ネットワークまたはリモートリソースでの fopen の使用は、php.ini でallow_url_fopen が有効になっている場合にのみ機能します。詳細については、ドキュメントを確認してください。

それで、ストリームリソースを手に入れました。それを使って何ができるでしょうか?

fwrite を使用してファイルストリームに書き込む

fwrite(resource $stream, string $data, ?int $length = null): int|false

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fwrite を使用すると、$data に提供されたコンテンツをストリームに書き込むことができます。 $length が指定された場合は、指定されたバイト数のみが書き込まれます。例を見てみましょう:

$fileStream = fopen('/tmp/test', 'w');

fwrite($fileStream,  "The quick brown fox jumps over the lazy dog", 10);

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この例では、$length = 10 を指定したため、コンテンツの一部のみ (「クイック」) が書き込まれ、残りは無視されます。

$mode = 'w' を使用してファイルストリームを開いたことに注目してください。これにより、ファイルにコンテンツを書き込むことができるようになりました。代わりに、$mode = 'r' でファイルを開いた場合は、「fwrite(): Write of 8192 bytes failed with errno=9 Bad file descriptor.

」のようなメッセージが表示されます。

コンテンツ全体をファイルストリームに書き込む別の例を見てみましょう:

$fileStream = fopen('/tmp/test', 'w');

fwrite($fileStream,  "The quick brown fox jumps over the lazy dog");

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$length を指定していないため、コンテンツ全体がファイルに書き込まれました。

ストリームに書き込むと、読み取り/書き込みポインターの位置がシーケンスの末尾に移動します。この場合、ストリームに書き込まれる文字列は 44 文字であるため、ポインタの位置は 43 になるはずです。

fwrite は、ファイルへの書き込みに加えて、ソケットなどの他のタイプのストリームにも書き込むことができます。ドキュメントから抜粋した例:

$sock = fsockopen("ssl://secure.example.com", 443, $errno, $errstr, 30);
if (!$sock) die("$errstr ($errno)\n");

$data = "foo=" . urlencode("Value for Foo") . "&bar=" . urlencode("Value for Bar");

fwrite($sock, "POST /form_action.php HTTP/1.0\r\n");
fwrite($sock, "Host: secure.example.com\r\n");
fwrite($sock, "Content-type: application/x-www-form-urlencoded\r\n");
fwrite($sock, "Content-length: " . strlen($data) . "\r\n");
fwrite($sock, "Accept: */*\r\n");
fwrite($sock, "\r\n");
fwrite($sock, $data);

$headers = "";
while ($str = trim(fgets($sock, 4096)))
$headers .= "$str\n";

echo "\n";

$body = "";
while (!feof($sock))
$body .= fgets($sock, 4096);

fclose($sock);

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fread を使用したストリームの読み取り

fread(resource $stream, int $length): string|false

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fread を使用すると、現在の読み取りポインターから開始して、ストリームから最大 $length バイトを読み取ることができます。例でわかるように、バイナリセーフであり、ローカルおよびネットワークのリソースで動作します。

fread を連続して呼び出すと、チャンクが読み取られ、読み取りポインタがこのチャンクの末尾に移動します。例として、前の例で記述されたファイルを考慮します:

# Content: "The quick brown fox jumps over the lazy dog"
$fileStream = fopen('/tmp/test', 'r');

echo fread($fileStream, 10) . PHP_EOL;      // 'The quick '
echo ftell($fileStream); // 10
echo fread($fileStream, 10) . PHP_EOL;      // 'brown fox '
echo ftell($fileStream); // 20

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すぐに ftell に戻りますが、これが行うことは単に読み取りポインターの現在位置を返すだけです。

次のいずれかの状況が発生するとすぐに読み取りが停止します (false を返します) (ドキュメントからのコピーです。後で理解します)。

length bytes have been read

EOF (end of file) is reached

a packet becomes available or the socket timeout occurs (for network streams)

if the stream is read buffered and it does not represent a plain file, at most one read of up to a number of bytes equal to the chunk size (usually 8192) is made; depending on the previously buffered data, the size of the returned data may be larger than the chunk size.

I don't know if you had the same felling, but this last part is pretty cryptic, so let's break it down.

"if the stream is read buffered"

Stream reads and writes can be buffered, that is, the content may be stored internally. It is possible to disable/enable the buffering, as well as set their sizes using stream_set_read_buffer and stream-set-write-buffer, but according to this comment on the PHP doc's Github, the description of these functions can be misleading.

This is where things get interesting, as this part of the documentation is really obscure. As per the comment, setting stream_set_read_buffer($stream, 0) would disable the read buffering, whereas stream_set_read_buffer($stream, 1) or stream_set_read_buffer($stream, 42) would simply enable it, ignoring its size (depending on the stream wrapper, which can override this default behaviour).

"... at most one read of up to a number of bytes equal to the chunk size (usually 8192) is made"

The chunk size is usually 8192 bytes or 8 KiB, as we will confirm in a bit. We can change this value using stream_set_chunk_size. Let's see it in action:

$f = fopen('https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/x86_64/alpine-standard-3.20.2-x86_64.iso', 'rb');

$previousPos = 0;
$chunkSize = 1024;
$i = 1;

while ($chunk = fread($f, $chunkSize)) {
    $bytesRead = (ftell($f) - $previousPos);
    $previousPos = ftell($f);

    echo "Iteration: {$i}. Bytes read: {$bytesRead}" . PHP_EOL;

    $i++;
}

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Output:

Iteration: 1. Bytes read: 1024
Iteration: 2. Bytes read: 1024
Iteration: 3. Bytes read: 1024
...
Iteration: 214016. Bytes read: 1024
Iteration: 214017. Bytes read: 169

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What happened in this case was clear:

We wanted up to 1024 bytes in each fread call and that's what we got
In the last call there were only 169 bytes remainder, which were returned
When there was nothing else to return, that is, EOF was reached fread returned false and the loop finished.

Now let's increase considerably the length provided to fread to 1 MiB and see what happens:

$f = fopen('https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/x86_64/alpine-standard-3.20.2-x86_64.iso', 'rb');

$previousPos = 0;
$chunkSize = 1048576; // 1 MiB
$i = 1;

while ($chunk = fread($f, $chunkSize)) {
    $bytesRead = (ftell($f) - $previousPos);
    $previousPos = ftell($f);

    echo "Iteration: {$i}. Bytes read: {$bytesRead}" . PHP_EOL;

    $i++;
}

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Output:

Iteration: 1. Bytes read: 1378
Iteration: 2. Bytes read: 1378
Iteration: 3. Bytes read: 1378
...
Iteration: 24. Bytes read: 1074
Iteration: 25. Bytes read: 8192
Iteration: 26. Bytes read: 8192
...
Iteration: 26777. Bytes read: 8192
Iteration: 26778. Bytes read: 8192
Iteration: 26779. Bytes read: 293

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So, even though we tried to read 1 MiB using fread, it read up to 8192 bytes - same value that the docs said it would. Interesting. Let's see another experiment:

$f = fopen('https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/x86_64/alpine-standard-3.20.2-x86_64.iso', 'rb');

$previousPos = 0;
$chunkSize = 1048576; // 1 MiB
$i = 1;

stream_set_chunk_size($f, $chunkSize); // Just added this line

while ($chunk = fread($f, $chunkSize)) {
    $bytesRead = (ftell($f) - $previousPos);
    $previousPos = ftell($f);

    echo "Iteration: {$i}. Bytes read: {$bytesRead}" . PHP_EOL;

    $i++;
}

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And the output:

Iteration: 1. Bytes read: 1378
Iteration: 2. Bytes read: 1378
Iteration: 3. Bytes read: 1378
...
Iteration: 12. Bytes read: 533
Iteration: 13. Bytes read: 16384
Iteration: 14. Bytes read: 16384
...
Iteration: 13386. Bytes read: 16384
Iteration: 13387. Bytes read: 16384
Iteration: 13388. Bytes read: 13626

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Notice that now fread read up to 16 KiB - not even close to what we wanted, but we've seen that stream_set_chunk_size did work, but there are some hard limits, that I suppose that depends also on the wrapper. Let's put that in practice with another experiment, using a local file this time:

$f = fopen('alpine-standard-3.20.2-x86_64.iso', 'rb');

$previousPos = 0;
$chunkSize = 1048576; // 1 MiB
$i = 1;

while ($chunk = fread($f, $chunkSize)) {
    $bytesRead = (ftell($f) - $previousPos);
    $previousPos = ftell($f);

    echo "Iteration: {$i}. Bytes read: {$bytesRead}" . PHP_EOL;

    $i++;
}

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Output:

Iteration: 1. Bytes read: 1048576
Iteration: 2. Bytes read: 1048576
...
Iteration: 208. Bytes read: 1048576
Iteration: 209. Bytes read: 1048576

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Aha! So using the local file handler we were able to fread 1 MiB as we wanted, and we did not even need to increase the buffer/chunk size with stream_set_chunk_size.

Wrapping up

I think that now the description is less cryptic, at least. Let's read it again (with some interventions):

if the stream is read buffered ...

and it does not represent a plain file (that is, local, not a network resource), ...

at most one read of up to a number of bytes equal to the chunk size (usually 8192) is made (and in our experiments we could confirm that this is true, at least one read of the chunk size was made); ...

depending on the previously buffered data, the size of the returned data may be larger than the chunk size (we did not experience that, but I assume it may happen depending on the wrapper).

There is definitely some room to play here, but I will challenge you. What would happen if you disable the buffers while reading a file? And a network resource? What if you write into a file?

ftell

ftell(resource $stream): int|false

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ftell returns the position of the read/write pointer (or null when the resource is not valid).

# Content: "The quick brown fox jumps over the lazy dog"
$fileStream = fopen('/tmp/test', 'r');

fread($fileStream, 10); # "The quick "
echo ftell($fileStream); 10

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stream_get_meta_data

stream_get_meta_data(resource $stream): array

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stream_get_meta_data returns information about the stream in form of an array. Let's see an example:

# Content: "The quick brown fox jumps over the lazy dog"
$fileStream = fopen('/tmp/test', 'r');
var_dump(stream_get_meta_data($fileStream)):

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The previous example would return in something like this:

array(9) {
  ["timed_out"]=>
  bool(false)
  ["blocked"]=>
  bool(true)
  ["eof"]=>
  bool(false)
  ["wrapper_type"]=>
  string(9) "plainfile"
  ["stream_type"]=>
  string(5) "STDIO"
  ["mode"]=>
  string(1) "r"
  ["unread_bytes"]=>
  int(0)
  ["seekable"]=>
  bool(true)
  ["uri"]=>
  string(16) "file:///tmp/test"
}

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This function's documentation is pretty honest describing each value ;)

fseek

fseek(resource $stream, int $offset, int $whence = SEEK_SET): int

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fseek sets the read/write pointer on the opened stream to the value provided to $offset.
The position will be updated based on $whence:

SEEK_SET: Position is set to $offset, that is, if you call
SEEK_CUR: Position is set based on the current one, that is, current + $offset
SEEK_END: Position is set to End Of File + $offset.

Using SEEK_END we can provide a negative value to $offset and go backwards from EOF. Its return value can be used to assess if the position has been set successfully (0) or has failed (-1).

Let's see some examples:

# Content: "The quick brown fox jumps over the lazy dog\n"
$fileStream = fopen('/tmp/test', 'r+');

fseek($fileStream, 4, SEEK_SET);
echo fread($fileStream, 5);         // 'quick'
echo ftell($fileStream);            // 9

fseek($fileStream, 7, SEEK_CUR);
echo ftell($fileStream);            // 16, that is, 9 + 7
echo fread($fileStream, 3);         // 'fox'  

fseek($fileStream, 5, SEEK_END);    // Sets the position past the End Of File
echo ftell($fileStream);            // 49, that is, EOF (at 44th position) + 5
echo fread($fileStream, 3);         // ''  
echo ftell($fileStream);            // 49, nothing to read, so read/write pointer hasn't changed
fwrite($fileStream, 'foo');
ftell($fileStream);                 // 52, that is, previous position + 3
fseek($fileStream, -3, SEEK_END);
ftell($fileStream);                 // 49, that is, 52 - 3
echo fread($fileStream, 3);         // 'foo'

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Some important considerations

As we've seen in this example, it is possible we seek past the End Of File and even read in an unwritten area (which returns 0 bytes), but some types of streams do not support it.
An important consideration is that not all streams can be seeked, for instance, you cannot fseek a remote resource:

$f = fopen('https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/x86_64/alpine-standard-3.20.2-x86_64.iso', 'rb');
fseek($f, 10); WARNING  fseek(): Stream does not support seeking

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This obviously makes total sense, as we cannot "fast-forward" and set a position on a remote resource. The stream in this case is only read sequentially, like a cassette tape.
We can determine if the stream is seekable or not via the seekable value returned by stream_get_meta_data that we've seen before.

We can fseek a resource opened in append mode (a or a+), but the data will always be appended.

rewind

rewind(resource $stream): bool

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This is a pure analogy of rewinding a videotape before returning it to video store. As expected, rewind sets the position of the read/write pointer to 0, which is basically the same as calling fseek with $offset 0.

The same considerations we've seen for fseek applies for rewind, that is:

You cannot rewind an unseekable stream
rewind on a resource opened in append mode will still write from the current position - the write pointer is not updated.

How about file_get_contents?

So far we've been working directly with resources. file_get_contents is a bit different, as it accepts the file path and returns the whole file content as a string, that is, it implicitly opens the resource.

file_get_contents(
   string $filename,
   bool $use_include_path = false,
   ?resource $context = null,
   int $offset = 0,
   ?int $length = null
): string|false

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Similar to fread, file_get_contents can work on local and remote resources, depending on the $filename we provide:

# Content: "The quick brown fox jumps over the lazy dog"

echo file_get_contents('/tmp/test'); // "The quick brown fox jumps over the lazy dog\n"

echo file_get_contents('https://www.php.net/images/logos/php-logo.svg'); // "<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 -1 100 50">\n ..."

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With $offset we can set the starting point to read the content, whereas with length we can get a given amount of bytes.

# Content: "The quick brown fox jumps over the lazy dog"
echo file_get_contents('/tmp/test', offset: 16, size: 3); // 'fox'

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offset also accepts negative values, which counts from the end of the stream.

# Content: "The quick brown fox jumps over the lazy dog"
echo file_get_contents('/tmp/test', offset: -4, size: 3); // 'dog'

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Notice that the same rules that govern fseek are also applied for $offset, that is - you cannot set an $offset while reading remote files, as the function would be basically fseek the stream, and we've seen that it does not work well.

The parameter context makes file_get_contents really flexible, enabling us set, for example:

Set a different HTTP method, such as POST instead of the default GET
Provide headers and content to a POST or PUT request
Disable SSL verification and allow self-signed certificates

We create a context using stream_context_create, example:

$context = stream_context_create(['http' => ['method' => "POST"]]);
file_get_contents('https://a-valid-resource.xyz', context: $context);

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You can find the list of options you can provide to stream_context_create in this page.

$networkResource = fopen('https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso', 'r');

while ($chunk = fread($networkResource, 1024)) {
   doSomething($chunk);
}

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Which one to use? fread, file_get_contents, fgets, another one?

The list of functions that we can use to read local or remote contents is lengthy, and each function can be seen as a tool in your tool belt, suitable for a specific purpose.

According to the docs, file_get_contents is the preferred way of reading contents of a file into a string, but, is it appropriate for all purposes?

What if you know that the content is large? Will it fit into memory?
Do you need it entirely in memory or can you work in chunks?
Are you going to work on local or remote files?

Ask yourself these (and other questions), make some performance benchmark tests and select the function that suits your needs the most.

PSR-7's StreamInterface

PSR defines the StreamInterface, which libraries such as Guzzle use to represent request and response bodies. When you send a request, the body is an instance of StreamInterface. Let's see an example, extracted from the Guzzle docs:

$client = new \GuzzleHttp\Client();
$response = $client->request('GET', 'http://httpbin.org/get');

$body = $response->getBody();
$body->seek(0);
$body->read(1024);

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I suppose that the methods available on $body look familiar for you now :D

StreamInterface implements methods that resemble a lot the functions we've just seen, such as:

seek()
tell()
eof()
read
write
isSeekable
isReadable()
isWritable
and so on.

Last but not least, we can use GuzzleHttp\Psr7\Utils::streamFor to create streams from strings, resources opened with fopen and instances of StreamInterface:

use GuzzleHttp\Psr7;

$stream = Psr7\Utils::streamFor('string data');
echo $stream;                   // string data
echo $stream->read(3);          // str
echo $stream->getContents();    // ing data
var_export($stream->eof());     // true
var_export($stream->tell());    // 11

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Summary

In this article we've seen what streams really are, learned how to create them, read from them, write to them, manipulate their pointers as well as clarified some obscured parts regarding read a write buffers.

If I did a good job, some of the doubts you might have had regarding streams are now a little bit clearer and, from now on, you'll write code more confidently, as you know what you are doing.

Should you noticed any errors, inaccuracies or there is any topic that is still unclear, let me know in the comments and I'd be glad to try to help.

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