Should flush() send headers when output buffering is enabled?

[7-zete-7]

I encountered a situation where the flush() function call sends headers, although the ob_get_level() function returns 2 at that moment.

Example:

headers_sent(); // => false
ob_start();
ob_start();
ob_get_level(); // => 2
headers_sent(); // => false
flush();
ob_get_level(); // => 2
headers_sent(); // => true
// ...

Headers were not sent before version 1.7.0, they started to be sent with version 1.7.0. I will trace this behavior back to #1622.

Is this correct behavior for the flush() function?

[withinboredom]

It's a good question. I'm more curious why PHP is telling the SAPI to flush at that point, if it is still output buffering. So, I'm inclined to say it is working properly, since PHP told us to flush our buffers, but more investigation might be needed.

[withinboredom]

I think you may be looking for ob_flush(): https://www.php.net/manual/en/function.ob-flush.php?

[7-zete-7]

No, I mean the flush() function.

The most interesting thing is that if you pass the following anonymous function to the second call of the ob_start() function

ob_start(fn () => '');

, then the call of the flush() function does not result in sending headers.

I encountered this problem while analyzing one of the working projects. The flush tag is used in the Twig template. No errors occur on the prod environment, while during development the error Warning: Cannot modify header information - headers already sent occurs.

More detailed information about the error

{
  "type": "https://tools.ietf.org/html/rfc2616#section-10",
  "title": "An error occurred",
  "status": 500,
  "detail": "Warning: Cannot modify header information - headers already sent",
  "class": "ErrorException",
  "trace": [
    {
      "namespace": "",
      "short_class": "",
      "class": "",
      "type": "",
      "function": "",
      "file": "/app/vendor/symfony/http-foundation/Session/SessionUtils.php",
      "line": 52,
      "args": []
    },
    {
      "namespace": "Symfony\\Component\\HttpFoundation\\Session",
      "short_class": "SessionUtils",
      "class": "Symfony\\Component\\HttpFoundation\\Session\\SessionUtils",
      "type": "::",
      "function": "popSessionCookie",
      "file": "/app/vendor/symfony/http-kernel/EventListener/AbstractSessionListener.php",
      "line": 161,
      "args": [
        [
          "string",
          "PHPSESSID"
        ],
        [
          "object",
          "SensitiveParameterValue"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\HttpKernel\\EventListener",
      "short_class": "AbstractSessionListener",
      "class": "Symfony\\Component\\HttpKernel\\EventListener\\AbstractSessionListener",
      "type": "->",
      "function": "onKernelResponse",
      "file": "/app/vendor/symfony/event-dispatcher/Debug/WrappedListener.php",
      "line": 116,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Event\\ResponseEvent"
        ],
        [
          "string",
          "kernel.response"
        ],
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Debug\\TraceableEventDispatcher"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\EventDispatcher\\Debug",
      "short_class": "WrappedListener",
      "class": "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener",
      "type": "->",
      "function": "__invoke",
      "file": "/app/vendor/symfony/event-dispatcher/EventDispatcher.php",
      "line": 220,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Event\\ResponseEvent"
        ],
        [
          "string",
          "kernel.response"
        ],
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Debug\\TraceableEventDispatcher"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\EventDispatcher",
      "short_class": "EventDispatcher",
      "class": "Symfony\\Component\\EventDispatcher\\EventDispatcher",
      "type": "->",
      "function": "callListeners",
      "file": "/app/vendor/symfony/event-dispatcher/EventDispatcher.php",
      "line": 56,
      "args": [
        [
          "array",
          [
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ],
            [
              "object",
              "Symfony\\Component\\EventDispatcher\\Debug\\WrappedListener"
            ]
          ]
        ],
        [
          "string",
          "kernel.response"
        ],
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Event\\ResponseEvent"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\EventDispatcher",
      "short_class": "EventDispatcher",
      "class": "Symfony\\Component\\EventDispatcher\\EventDispatcher",
      "type": "->",
      "function": "dispatch",
      "file": "/app/vendor/symfony/event-dispatcher/Debug/TraceableEventDispatcher.php",
      "line": 139,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Event\\ResponseEvent"
        ],
        [
          "string",
          "kernel.response"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\EventDispatcher\\Debug",
      "short_class": "TraceableEventDispatcher",
      "class": "Symfony\\Component\\EventDispatcher\\Debug\\TraceableEventDispatcher",
      "type": "->",
      "function": "dispatch",
      "file": "/app/vendor/symfony/http-kernel/HttpKernel.php",
      "line": 214,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpKernel\\Event\\ResponseEvent"
        ],
        [
          "string",
          "kernel.response"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\HttpKernel",
      "short_class": "HttpKernel",
      "class": "Symfony\\Component\\HttpKernel\\HttpKernel",
      "type": "->",
      "function": "filterResponse",
      "file": "/app/vendor/symfony/http-kernel/HttpKernel.php",
      "line": 202,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpFoundation\\Response"
        ],
        [
          "object",
          "Symfony\\Component\\HttpFoundation\\Request"
        ],
        [
          "integer",
          1
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\HttpKernel",
      "short_class": "HttpKernel",
      "class": "Symfony\\Component\\HttpKernel\\HttpKernel",
      "type": "->",
      "function": "handleRaw",
      "file": "/app/vendor/symfony/http-kernel/HttpKernel.php",
      "line": 76,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpFoundation\\Request"
        ],
        [
          "integer",
          1
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\HttpKernel",
      "short_class": "HttpKernel",
      "class": "Symfony\\Component\\HttpKernel\\HttpKernel",
      "type": "->",
      "function": "handle",
      "file": "/app/vendor/symfony/http-kernel/Kernel.php",
      "line": 197,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpFoundation\\Request"
        ],
        [
          "integer",
          1
        ],
        [
          "boolean",
          true
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\HttpKernel",
      "short_class": "Kernel",
      "class": "Symfony\\Component\\HttpKernel\\Kernel",
      "type": "->",
      "function": "handle",
      "file": "/app/vendor/symfony/runtime/Runner/Symfony/HttpKernelRunner.php",
      "line": 35,
      "args": [
        [
          "object",
          "Symfony\\Component\\HttpFoundation\\Request"
        ]
      ]
    },
    {
      "namespace": "Symfony\\Component\\Runtime\\Runner\\Symfony",
      "short_class": "HttpKernelRunner",
      "class": "Symfony\\Component\\Runtime\\Runner\\Symfony\\HttpKernelRunner",
      "type": "->",
      "function": "run",
      "file": "/app/vendor/autoload_runtime.php",
      "line": 29,
      "args": []
    },
    {
      "namespace": "",
      "short_class": "",
      "class": "",
      "type": "",
      "function": "require_once",
      "file": "/app/public/index.php",
      "line": 14,
      "args": [
        [
          "string",
          "/app/vendor/autoload_runtime.php"
        ]
      ]
    }
  ]
}

The investigation led me to a line in the Twig template with the flush tag (which is equivalent to calling the flush() function). Calling the ob_get_level() function before and after flush returns 2 (one from Symfony and one from Twig). Calling the headers_sent() function before flush returns false, and after flush returns true.

Further investigation into the reasons why everything works without errors on the prod environment led to the Twig\Template::render() method. This method calls the ob_start() function, but its calls differ for debug and non-debug environments.

No changes were made to the project before the error appeared. Not a single library was updated either.

I started trying possible combinations of PHP and FrankenPHP versions, and managed to find out that the problem only occurs when migrating from FrankenPHP version 1.6.2 to version 1.7.0. As a result of the investigation, I found the #1622.

I do not have the necessary competence in the correctness of the flush() function, so I am interested in whether the previous behavior was an error or the current behavior is an error.

[withinboredom]

After testing the following on nginx + fpm and on frankenphp; the behaviour is indeed correct:

    headers_sent();
    ob_start();
    ob_start();
    var_dump(headers_sent()); // false
    flush();
    header("Content-Type: text/plain"); // notice: headers already sent
    var_dump(headers_sent()); // true

[7-zete-7]

I re-tested and found that the flush() function behavior does indeed send headers in any case (even if ob_start(fn () => '') was called).

I used the following script to test:

<?php

# Choose one of output buffer filters:
ob_start(); dump_state();
#ob_start(fn () => ''); dump_state();
#ob_start(function () {}); dump_state();

echo 'Hello, World!'; dump_state();

flush(); dump_state();

ob_end_clean(); dump_state();

function dump_state(): void
{
    error_log(sprintf('__LINE__ => %d; headers_sent() => %d; ob_get_level() => %d', debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS)[0]['line'], headers_sent(), ob_get_level()));
}

For all output buffer filter variants, both for FrankenPHP (^1.7) and PHP-FPM (^8.4) the behavior is the same.

Thanks @withinboredom for helping me figure this out!