Description:
------------
See test script: when returning a undefined key from stream_stat(), the behavior 
of the Zend interpretor changes.

I confirmed this on PHP 5.3.5, pjoye reproduced it separately.

Test script:
---------------
--TEST--
Test that user space streams do not break the fabric of the universe.
--FILE--
<?php

class UserSpaceStream {
  public function stream_open($uri, $mode, $options, &$opened_path) {
    return TRUE;
  }

  public function stream_read($count) {
    return '';
  }

  public function stream_write($data) {
    return TRUE;
  }

  public function stream_eof() {
    return FALSE;
  }

  public function stream_stat() {
    $stat = array();

    // This triggers (correctly) a "Notice: Undefined index: size in %s on line %d".
    $array = array();
    $stat['size'] = $array['size'];

    return $stat;
  }

}

stream_wrapper_register("test", "UserSpaceStream");

// Alone, the assignment of a key to an undefined variable is legal.
$undefined_variable['key'] = 'value';

// This fstat() call triggers something weird...
$h = fopen('test://anyfile', 'r');
fstat($h);

// ... from this point, PHP triggers a "Warning: Cannot use a scalar value as
// an array" for each assignement of array keys to undefined variables.
$undefined_variable2['key'] = 'value';

?>
--EXPECTF--
Notice: Undefined index: size in %s on line %d