TYPO3 Core

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51915

The reason for this error in PHP 7.1 is only in the case when $queryString is empty:

The empty index operator is not supported for strings anymore
Applying the empty index operator to a string (e.g. $str[] = $x) throws a fatal error instead of converting silently to array.
Ref.: https://secure.php.net/manual/en/migration71.incompatible.php#migration71.incompatible.empty-string-index-operator

Probably more to the point:

...As of PHP 7.1.0, assigning an empty string throws a fatal error. Formerly, it assigned a NULL byte.
Ref.: https://secure.php.net/manual/en/language.types.string.php#language.types.string.substr

Examples of the above for PHP 7.0 and PHP 7.1 respectively:

PHP 7.0:

$ php -a
Interactive shell

php > $a = '';
php > $a[] = 'b';
php >

PHP 7.1:

$ php -a
Interactive shell

php > $a = '';
php > $a[] = 'b';

Warning: Uncaught Error: [] operator not supported for strings in php shell code:1
Stack trace:
#0 {main}
  thrown in php shell code on line 1
php > 

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51915

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51915

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51915

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51915

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51915

Patch set 1 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51944