Description:
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I am having a for loop which is iterating through a large array $result. Every array item $row is used to generate a new line of string $line. This string is concatenated into $str. To preserve memory, I am using unset($result[$i]) to clear every array item that has been processed in the original array.

This (more or less exotic piece of code) works unless the string exceeds a length of 999'775 characters. In this case the return value is just empty. This is highly irritating because:

1. The calculation effort (in this example do_something()) is not a problem. By using `echo count($result).' - '.strlen($str)."\n"` I can see that the loop finishes properly. There is no web server or php error shown.

2. The memory_limit is also not a problem. I am working with more data on other parts of the application.

It appears that the problem lies in return $str itself. If I am using `return substr($str, 0, 980000)` then it just works fine.

I can't put those exceeding return value strings into another string variable. But I am able to do a strlen() to get a proper result (1310307). But I can't use them properly as a string. Using an `echo $str` in the function works properly. But return doesn't.

Test script:
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function what_is_going_on($result){
   $resultcount = count($result);
   for($i=0; $i<$resultcount; ++$i){
      $row = $result[$i];

      $line = do_something($row);
      $str.= $line;

      unset($result[$i]);
   }

   return $str;
}

$output = what_is_going_on($result);
echo $output;