Description:
------------
When executing the code below with "?lang=de_DE", the first call will result in a float with comma in the concatenated string. On the second call this will result in a float with a dot.

When disabling opcache the issue is gone and it is always a comma.
Also in php 7.3 this issue does not occure.

So it seems that opcache is ignoring the setlocale.

Test script:
---------------
class Test {
    function __construct() {
        // call https://domain.tld?lang=de_DE
        // first call will result in a float with COMMA in the concatenated string
        // second call will result in a float with DOT in the concatenated string

        if ($_GET['lang']) {
            setlocale(LC_ALL, $_GET['lang'] . '.utf-8');
        }

        $z = 0.5;
        $y = $z . ' invalidate cache for this class by changing something here: v1';
        echo 'Concatenated string: ' . $y . '<br />';
        echo 'Plain Variable: ';
        echo $z;
        echo '<br />';
        exit;
    }
}

$test = new Test();

Expected result:
----------------
The float will always be with ",".