Description:
------------
[opcache]
opcache.enable=1
opcache.enable_cli=1
opcache.interned_strings_buffer=8
opcache.max_accelerated_files=4000

# php -v
PHP 7.3.1-1+0~20190113101756.25+stretch~1.gbp15aaa9 (cli) (built: Jan 13 2019 10:17:57) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.1, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.1-1+0~20190113101756.25+stretch~1.gbp15aaa9, Copyright (c) 1999-2018, by Zend Technologies




Test script:
---------------
function testB(){
	$x = ['a' => '', 'b' => '']; 
	$num = range(0, 9); 
	foreach($num as $k){
		$x['a'] .= $k;
	}
	return $x;
}
function testA(){
	$x = ['a' => '', 'b' => '']; 
	$num = range(0, 9); 
	foreach($num as $k){
		$x['a'] .= $k;
	}
	return $x['a'];
}
// if opcache.enable=1 => string(0) ""
// if opcache.enable=0 => string(10) "0123456789"
// if call first testB() => string(10) "0123456789"
//var_dump(testB());
var_dump(testA());

full script: https://gist.github.com/poiuty/9759972fcc6bee42bbb250c03ef43e77