PHP Output Buffer bug

test-ob.php

<?php

$path = "/tmp/ob.log";
$logHandle = fopen($path, 'w') or die("unable to open temp file");

ob_start(function($buffer) use ($logHandle) { fwrite($logHandle, $buffer); }, 20);

echo str_repeat("0", 20); // fill in the buffer

for($i = 0; $i < 10; $i++) {
	echo str_pad($i, 9, ' ', STR_PAD_LEFT) . "\n"; // full buffer dumped every time
}

ob_end_flush();
fclose($logHandle);

printf("Output file size: %d, expected %d\n", filesize($path), 20 + 10 * 10);