Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

How about a repro that doesn't rely on your test framework?

<?php
class Base {
    protected static $prop = 'BASE';
    public function getProp() {
        return static::$prop;
    }
}
class Mock extends Base { }

$obj = new Mock;
$excepted = 'REFLECTED';

$property = new ReflectionProperty('Base', 'prop');
$property->setAccessible(true);
$property->setValue($excepted);

$actual = $obj->getProp();
echo "Schould be `{$excepted}` and is `{$actual}`\n";
// exit($actual !== $excepted ? 1 : 0);
?>

proof: https://travis-ci.org/majkel89/php7-bug

https://3v4l.org/nvhZQ

I spent some time playing with this bug and it turned out to be worse problem that it originally seemed. Changing value of static property with reflection **permanently breaks the inheritance chain**.

https://3v4l.org/T43Sh

Could someone look into this?

This was fixed in 7.0.1, someone forgot to update the bug.