PHP5 incompatibility with isset() on ArrayObjects

[whatthejeff]

This is related to #2181 and #2871.

<?php

class TestArray extends ArrayObject 
{
    public function offsetExists($index) 
    {
        return true;
    }
}

$test = new TestArray();
var_dump(isset($test['abc']));

In Zend PHP you get the following:

bool(true)

And in HHVM you get the following:

Notice: Undefined index: abc in...
bool(false)

[filips123]

Is this fixed? There is still similar error with ArrayAccess. See https://stackoverflow.com/questions/51697914/php-5-6-arrayaccess-function-isset-calls-offsetget-and-causes-undefined-index and https://3v4l.org/7C2Fs.