It looks like you're using Sendmail-brand SENDMAIL(tm), which I've not used in quite awhile. But, at least with Exim-brand sendmail, it does work OK. (See snippets below.)
You might want to check your mail log (usually /var/log/mail.log or thereabouts) to see if it is reporting any errors; it's possible that your application is running as a user which isn't allowed to use the -f option, and it should log about that. Alternatively, you might be editing the wrong php.ini file: it loads different files depending on whether you're using command line php, mod_php, or PHP-FPM.
If you try 'sendmail -t -i -f...@...' from the command line, that would at least eliminate the MTA itself as a problem (but it could still be disallowing -f for other users); it'll sit and await your input, which should consist of a few headers (most importantly, To:), followed by one blank line, followed by a message, and then followed by ctrl-D on a line of its own. Then, you can check what comes out the other end.
Code:
rtucker@witte:~$ sendmail -t -i
Subject: test message 1
To: rtucker@gmail.com
X-Test: test header 1
this is with the default
Code:
Delivered-To: rtucker@gmail.com
Received: by 10.182.103.198 with SMTP id fy6csp168424obb;
Sat, 12 Jan 2013 07:32:32 -0800 (PST)
X-Received: by 10.236.147.204 with SMTP id t52mr89426823yhj.9.1358004752654;
Sat, 12 Jan 2013 07:32:32 -0800 (PST)
Return-Path: <rtucker@sodtech.net>
Received: from framboise.hoopycat.com (framboise.cdknnjln.sodtech.net. [2600:3c03::f03c:91ff:fe96:1dc9])
by mx.google.com with ESMTP id u19si4044443anh.133.2013.01.12.07.32.32;
Sat, 12 Jan 2013 07:32:32 -0800 (PST)
Received-SPF: pass (google.com: best guess record for domain of rtucker@sodtech.net designates 2600:3c03::f03c:91ff:fe96:1dc9 as permitted sender) client-ip=2600:3c03::f03c:91ff:fe96:1dc9;
Authentication-Results: mx.google.com;
spf=pass (google.com: best guess record for domain of rtucker@sodtech.net designates 2600:3c03::f03c:91ff:fe96:1dc9 as permitted sender) smtp.mail=rtucker@sodtech.net
Received: from witte.rochnyav.sodtech.net (unknown [IPv6:2001:470:8dde:2:b456:7b95:74a7:8068])
by framboise.hoopycat.com (Postfix) with ESMTPS id 4A885804A
for <rtucker@gmail.com>; Sat, 12 Jan 2013 15:32:32 +0000 (UTC)
Received: from rtucker by witte.rochnyav.sodtech.net with local (Exim 4.71)
(envelope-from <rtucker@witte.rochnyav.sodtech.net>)
id 1Tu348-0004g7-JW
for rtucker@gmail.com; Sat, 12 Jan 2013 10:32:26 -0500
Date: Sat, 12 Jan 2013 10:32:22 -0500
Message-Id: <E1Tu348-0004g7-JW@witte.rochnyav.sodtech.net>
Subject: test message 1
To: rtucker@gmail.com
X-Test: test header 1
From: Ryan Tucker <rtucker@sodtech.net>
this is with the default
Code:
rtucker@witte:~$ sendmail -t -i -frtucker@hoopycat.com
Subject: test message 2
To: rtucker@gmail.com
X-Test: test header 2
this is with a forced from address
Code:
Delivered-To: rtucker@gmail.com
Received: by 10.182.103.198 with SMTP id fy6csp168439obb;
Sat, 12 Jan 2013 07:32:58 -0800 (PST)
X-Received: by 10.101.136.11 with SMTP id o11mr21584144ann.26.1358004778297;
Sat, 12 Jan 2013 07:32:58 -0800 (PST)
Return-Path: <rtucker@hoopycat.com>
Received: from framboise.hoopycat.com (framboise.cdknnjln.sodtech.net. [2600:3c03::f03c:91ff:fe96:1dc9])
by mx.google.com with ESMTP id b70si9185183yhe.134.2013.01.12.07.32.58;
Sat, 12 Jan 2013 07:32:58 -0800 (PST)
Received-SPF: pass (google.com: domain of rtucker@hoopycat.com designates 2600:3c03::f03c:91ff:fe96:1dc9 as permitted sender) client-ip=2600:3c03::f03c:91ff:fe96:1dc9;
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of rtucker@hoopycat.com designates 2600:3c03::f03c:91ff:fe96:1dc9 as permitted sender) smtp.mail=rtucker@hoopycat.com
Received: from witte.rochnyav.sodtech.net (unknown [IPv6:2001:470:8dde:2:b456:7b95:74a7:8068])
by framboise.hoopycat.com (Postfix) with ESMTPS id 06544804A
for <rtucker@gmail.com>; Sat, 12 Jan 2013 15:32:58 +0000 (UTC)
Received: from rtucker by witte.rochnyav.sodtech.net with local (Exim 4.71)
(envelope-from <rtucker@hoopycat.com>)
id 1Tu34d-0004gO-Aj
for rtucker@gmail.com; Sat, 12 Jan 2013 10:32:57 -0500
Date: Sat, 12 Jan 2013 10:32:53 -0500
Message-Id: <E1Tu34d-0004gO-Aj@witte.rochnyav.sodtech.net>
Subject: test message 2
To: rtucker@gmail.com
X-Test: test header 2
From: Ryan Tucker <rtucker@hoopycat.com>
this is with a forced from address
_________________
Code:
/* TODO: need to add signature to posts */