This is a (multiple allowed):

• bug
• enhancement
• feature-discussion (RFC)
• CakePHP Version: 3.2.8
• Platform and Target: PHP 7 on Ubuntu Linux

What you did

Use $this->request->referer(true) when the referer is our home (/) crashes under PHP7 with the error Uninitialized string offset: 0.

The problem is in this line of the Request@referer method:

if ($local && strpos($ref, $base) === 0) {
    $ref = substr($ref, strlen($base));
    if ($ref[0] !== '/') {
        $ref = '/' . $ref;
    }
    return $ref;
}

$ref is an empty string due to the substr($ref, strlen($base)).

Expected Behavior

referer should be returning /

Actual Behavior

Crashes with the error Uninitialized string offset: 0.

I'm just creating a PR with the fix and its test. BTW, I'm not sure about how I've fixed, but let's discuss it on the PR.