PHP Causing Segmentation Fault & Apache Blank Response

I recently updated a Debian Lenny server to php 5.3,5 using the dotdeb source. Soon after doing so certain (but not all) sites on the server stopped responding to requests. A blank response would be returned – no headers, no content, nothing.

I found this related question on stackoverflow which seems to describe something similar and used the same code the user had in their answer to see if I could replicate the issue:

<?php

class A {
    public function __construct()
    {
        new B;
    }
}

class B {
    public function __construct()
    {
        new A;
    }
}

new A;

print 'Loaded Class A';

?>

This triggered the problem – the page returned absolutely nothing despite the original question stating this was fixed in PHP 5.5.0. No CPU block as you'd expect, no wait, just an almost instant zero response.

I then ran this same code from the cli (php -f test.php) and the only output I got was 'Segmentation fault'.

Tailing the kernel log I've spotted:

Feb 16 07:04:06 creature kernel: [192203.269037] php[17710] general protection ip:76ef37 sp:7fff155e9bb0 error:0 in php5[400000+870000]
Feb 16 08:57:31 creature kernel: [199639.699854] apache2[31136]: segfault at 7fff13a84fe0 ip 7f730514ea40 sp 7fff13a85008 error 6 in libphp5.so[7f7304ce8000+915000]

All extremely odd and I'm not sure what it's pointing to/what I should do to debug this further. As I said some sites work but code such as the above definitely trigger it. Not that the sites I want to server have code like that – it's just an example.

Any help is much appreciated!