How can I use var_dump + output buffering without memory errors?

Question

I\'m using a debugging aid in an application that uses var_dump() with output buffering to capture variables and display them. However, I\'m running into an iss

Answer 1

Well, if the physical memory is limited (you see the fatal error:)

Fatal error: Allowed memory size of 536870912 bytes exhausted

I would suggest to do the output buffering on disk (see callback parameter on ob_start). Output buffering works chunked, that means, if there still is enough memory to keep the single chunk in memory, you can store it into a temporary file.

// handle output buffering via callback, set chunksize to one kilobyte
ob_start($output_callback, $chunk_size = 1024);

However you must keep in mind that this will only prevent the fatal error while buffering. If you now want to return the buffer, you still need to have enough memory or you return the file-handle or file-path so that you can also stream the output.

However you can use that file then to obtain the size in bytes needed. The overhead for PHP strings is not much IIRC, so if there still is enough memory free for the filesize this should work well. You can substract offset to have a little room and play safe. Just try and error a little what it makes.

Some Example code (PHP 5.4):

chunkSize = $chunkSize;
        $this->store     = new SplTempFileObject();
    }

    public function start() {
        if ($this->started) {
            throw new BadMethodCallException('Buffering already started, can not start again.');
        }
        $this->started = true;
        $result = ob_start(array($this, 'bufferCallback'), $this->chunkSize);
        $this->verbose && file_put_contents('php://stderr', sprintf("Starting Buffering: %d; Level %d\n", $result, ob_get_level()));
        return $result;
    }

    public function flush() {
        $this->started && ob_flush();
    }

    public function stop() {
        if ($this->started) {
            ob_flush();
            $result = ob_end_flush();
            $this->started = false;
            $this->verbose && file_put_contents('php://stderr', sprintf("Buffering stopped: %d; Level %d\n", $result, ob_get_level()));
        }
    }

    private function bufferCallback($chunk, $flags) {

        $chunkSize = strlen($chunk);

        if ($this->verbose) {
            $level     = ob_get_level();
            $constants = ['PHP_OUTPUT_HANDLER_START', 'PHP_OUTPUT_HANDLER_WRITE', 'PHP_OUTPUT_HANDLER_FLUSH', 'PHP_OUTPUT_HANDLER_CLEAN', 'PHP_OUTPUT_HANDLER_FINAL'];
            $flagsText = '';
            foreach ($constants as $i => $constant) {
                if ($flags & ($value = constant($constant)) || $value == $flags) {
                    $flagsText .= (strlen($flagsText) ? ' | ' : '') . $constant . "[$value]";
                }
            }

            file_put_contents('php://stderr', "Buffer Callback: Chunk Size $chunkSize; Flags $flags ($flagsText); Level $level\n");
        }

        if ($flags & PHP_OUTPUT_HANDLER_FINAL) {
            return TRUE;
        }

        if ($flags & PHP_OUTPUT_HANDLER_START) {
            $this->store->fseek(0, SEEK_END);
        }

        $chunkSize && $this->store->fwrite($chunk);

        if ($flags & PHP_OUTPUT_HANDLER_FLUSH) {
            // there is nothing to d
        }

        if ($flags & PHP_OUTPUT_HANDLER_CLEAN) {
            $this->store->ftruncate(0);
        }

        return "";
    }

    public function getSize() {
        $this->store->fseek(0, SEEK_END);
        return $this->store->ftell();
    }

    public function getBufferFile() {
        return $this->store;
    }

    public function getBuffer() {
        $array = iterator_to_array($this->store);
        return implode('', $array);
    }

    public function __toString() {
        return $this->getBuffer();
    }

    public function endClean() {
        return ob_end_clean();
    }
}


$buffer  = new OutputBuffer();
echo "Starting Buffering now.\n=======================\n";
$buffer->start();

foreach (range(1, 10) as $iteration) {
    $string = "fill{$iteration}";
    echo str_repeat($string, 100), "\n";
}
$buffer->stop();

echo "Buffering Results:\n==================\n";
$size = $buffer->getSize();
echo "Buffer Size: $size (string length: ", strlen($buffer), ").\n";
echo "Peeking into buffer: ", var_dump(substr($buffer, 0, 10)), ' ...', var_dump(substr($buffer, -10)), "\n";

Output:

STDERR: Starting Buffering: 1; Level 1
STDERR: Buffer Callback: Chunk Size 1502; Flags 1 (PHP_OUTPUT_HANDLER_START[1]); Level 1
STDERR: Buffer Callback: Chunk Size 1503; Flags 0 (PHP_OUTPUT_HANDLER_WRITE[0]); Level 1
STDERR: Buffer Callback: Chunk Size 1503; Flags 0 (PHP_OUTPUT_HANDLER_WRITE[0]); Level 1
STDERR: Buffer Callback: Chunk Size 602; Flags 4 (PHP_OUTPUT_HANDLER_FLUSH[4]); Level 1
STDERR: Buffer Callback: Chunk Size 0; Flags 8 (PHP_OUTPUT_HANDLER_FINAL[8]); Level 1
STDERR: Buffering stopped: 1; Level 0
Starting Buffering now.
=======================
Buffering Results:
==================
Buffer Size: 5110 (string length: 5110).
Peeking into buffer: string(10) "fill1fill1"
 ...string(10) "l10fill10\n"