I had no problems with above script on a PHP 7.0.5 machine with xDebug, but my problem is reproducible with the following snippet:

<code>
<?php

ini_set('display_errors', 1);
error_reporting(E_ALL);

class myException extends \Exception {
public function __toString() {
return 'my own custom message'; // Produces segfault
return 'my own in house custom message'; // Returns: Fatal error: Uncaught my own in /vagrant/src/test.php on line 18
}
}

try {
throw new myException('Hello world!');
} catch (Exception $e) {
throw new myException('Bye world! (originally: '.$e->getMessage().')');
}
</code>

Effectively, adding " in " in the custom message "solves" the problem, however, notice how the returned message has been stripped, should have been:

Fatal error: Uncaught my own in house custom message in /vagrant/src/test.php on line 18

is:

Fatal error: Uncaught my own in /vagrant/src/test.php on line 18

When disabling display_errors no segfault occurs.

No segfault on latest PHP 5.6 with latest xDebug either, but the displayed uncaught fatal error by xDebug lacks a stacktrace, with or without the " in " part of the custom exception.

Thanks for your report! A fix for this will make it into 2.4.1.