Description:
------------
Under some circumstances, the following code (shortened, see link below for more info):

$selects = array();
foreach (array_keys($this->getAttributesByTable()) as $table) {
  echo gettype($selects);
  var_dump($selects);
  echo gettype($selects[$eavType]);

  $selects[$eavType][] = $this->_addLoadAttributesSelectFields($select, $table, $eavType);
}

Got executed with the following result:

array
.../Abstract.php:992: array (size=0)  empty
string

( ! ) Fatal error: Uncaught Error: [] operator not supported for strings in .../Abstract.php on line 995

As you can see, it seems like for some reason type of value in an empty array got recognized like a string. I checked a lot of options and asked for community help but as far nobody could answer why is that happening, so I decided to make a bug report.

Unfortunately, this happens very rarely under some unknown circumstances, so I can't make test script or make a gdb backtrace using CLI. If you can help me and explain how to grab some more useful data about scope or execution environment

Here's the extended discussion with more info: https://stackoverflow.com/questions/54913287/under-some-circumstances-operator-not-supported-for-strings-error-occurs-whe

Reproduced on both local (7.2.15, Ubuntu 16.04 x64), dev and production (7.2.15 RHEL 6) environments. Don't think It's related to environment or some modules, but here's the list of installed modules (tried to pick only additionally installed modules).

[PHP Modules]
...
gd
...
iconv
igbinary
imagick
...
mbstring
mcrypt
mysqli
mysqlnd
openssl
pcntl
pcre
PDO
pdo_mysql
Phar
posix
readline
redis
Reflection
session
shmop
SimpleXML
sockets
sodium
SPL
...
xdebug
xml
...
Zend OPcache
zip
zlib

[Zend Modules]
Xdebug
Zend OPcache

Excuse me for possible grammar errors and thanks in advance.