This is a very similar issue to bug #19280 (http://bugs.php.net/bug.php?id=19280), but that bug was closed and this issue still exists.

php4-STABLE-200302041230
imap-2001a (from RedHat 8.0)
apache-2.0.44 (happens from CLI too, so probably irrelevant)

also occurs with php-4.3.0, apache-2.0.43

Strangely constructed To: or From: header causes a segfault, even from CLI php.  Here are sample headers to reconstruct the problem:

---
From test@test.com  Mon Feb  3 14:08:06 2003
Return-Path: <test@test.com>
Date: Mon,  3 Feb 2003 14:08:08 -0500
From: PHP Test Case <test@test.com>
To: \\\\\\\\\\\\\\\\\\\ <t@t.com>
Subject: test 4
MIME-Version: 1.0
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
---

Swapping the To: and From: headers causes the same problem.

The following script will reproduce the segfault:

---
<?php

    $mailbox = imap_open(
        "{127.0.0.1:143/imap/notls}"."test",
        "rob", "******", OP_READONLY);

    $header = imap_header($mailbox, 1);
    echo $header->toaddress . "\n";
    echo $header->fromaddress . "\n";

    imap_close($mailbox);
?>
---

Notes:

In ext/imap/php_imap.c, if I change the define:
#define PHP_IMAP_ADDRESS_SIZE_BUF 10
to something larger, say 100, the segfault disappears.  Since PHP_IMAP_ADDRESS_SIZE_BUF was added to solve bug #19280 I thought this may be a related issue.

The badly constructed address above (\\\\\\\\\\\\\\\\\\\ <t@t.com>) was the shortest test case I could come up with.  Note that reducing the number of backslashes makes it work, but adding the same number of alphanumeric characters back into the email address itself (between the <>) breaks it again.

An older server running PHP 4.1.2, imap-2001a, Apache 1.3.27, Horde/IMP, was not affected by the same messages.

Thanks for looking into this!  Let me know if you need additional information.  PHP ./configure line pasted below:

'./configure' '--prefix=/usr/local/php4-STABLE-200302041230' '--with-config-file-path=/etc' '--enable-force-cgi-redirect' '--disable-debug' '--enable-pic' '--disable-rpath' '--enable-inline-optimization' '--with-bz2' '--with-db4' '--with-zlib' '--with-gd' '--with-png-dir=/usr' '--with-freetype-dir=/usr' '--enable-gd-native-ttf' '--with-ttf' '--with-gdbm' '--with-gettext' '--with-ncurses' '--with-gmp' '--with-iconv' '--with-jpeg-dir=/usr' '--with-png' '--with-pspell' '--with-regex=system' '--with-xml' '--with-expat-dir=/usr' '--enable-bcmath' '--enable-exif' '--enable-ftp' '--enable-magic-quotes' '--enable-safe-mode=no' '--enable-sockets' '--enable-sysvsem' '--enable-sysvshm' '--enable-discard-path' '--enable-track-vars' '--enable-trans-sid' '--without-oci8' '--with-pear=/usr/share/pear' '--with-imap=shared' '--with-imap-ssl' '--with-ldap=shared' '--with-mysql=shared,/usr' '--enable-memory-limit' '--enable-bcmath' '--enable-shmop' '--enable-versioning' '--with-apxs2=/usr/local/apache-2.0.44/bin/apxs' '--with-kerberos=/usr/kerberos'