imap_headerinfo

When I was testing imap_headerinfo() with an e-mail that had multiple recipients (multiple e-mails in to to: and/or cc: field), I noticed that imap_headerinfo() was failing hard for me on PHP 5.2.10-2ubuntu6.4 with Suhosin-Patch 0.9.7 (cli).
Rather than providing me an array with each and every e-mail address listed in the to and/or cc fields, it was only providing me the first listed. This was disappointing.
 [to] => Array
 ( 
 [0] => stdClass Object
 ( 
 [mailbox] => game
 [host] => blunts.com
 )
 )
Luckily, there was a cool workaround to this problem:
imap_rfc822_parse_headers(imap_fetchheader( string ); which subsequentally worked like a nice little pet would:
 [to] => Array
 ( 
 [0] => stdClass Object
 ( 
 [mailbox] => game
 [host] => blunts.com
 )
 [1] => stdClass Object
 ( 
 [mailbox] => dutch
 [host] => masters.com
 )
 )
TL;DR: 
So in other words, instead of imap_headerinfo() use imap_rfc822_parse_headers(imap_fetchheader()).
Hope this helps anyone else that runs into this issue from now into the future. This post was suggested by people in #PHP on FreeNode IRC.

I'm not entirely sure why this is, but if you loop through all of the messages in a mailbox, calling imap_header() each time, you can significantly increase performance by calling imap_headers() first.
Compare this:
<?php
 $imap = imap_open("{my.server.com:143}INBOX", "user", "pass");
 $n_msgs = imap_num_msg($imap);
 $s = microtime(true);
 for ($i=0; $i<$n_msgs; $i++) {
 $header = imap_header($imap, $i);
 }
 $e = microtime(true);
 echo ($e - $s);
 imap_close($imap);
?>

With this:
<?php
 $imap = imap_open("{my.server.com:143}INBOX", "user", "pass");
 $n_msgs = imap_num_msg($imap);
 /****** adding this line: ******/
 imap_headers($imap)
 /***************************/
 $s = microtime(true);
 for ($i=0; $i<$n_msgs; $i++) {
 $header = imap_header($imap, $i);
 }
 $e = microtime(true);
 echo ($e - $s);
 imap_close($imap);
?>

The performance difference, as I have tested on several boxes, connecting to several different servers, is that the second code snippet ALWAYS takes 1/2 the time, if not less.
Perhaps it is because imap_headers() retrieves all of the messages on one connection, whereas imap_header() has to make a new fetch request for each message?? I'm not sure WHY it is faster if imap_headers() is called first, but I do know that it is, so I thought I'd pass on the knowledge. If anyone knows why this is, please let me know....

Please Note : imap_headerinfo only returns a subset of the headers, rather than the entire thing.
Among other things, this means it only shows the first recipient from the "to" section of the email, rather than all recipients.
If you're not seeing something you expected to, you'll be better off using
$hdr_raw = imap_fetchheader($mbox, $mailid);
$hdr = imap_rfc822_parse_headers($hdr_raw);
then you'll see the full set of headers, and be able to do more with it.

If you want to extract values from to, from, or other header elements, they are an object and you need to loop over them i.e.
$header = imap_header($mbox, $message_id);
$from = $header->from;
foreach ($from as $id => $object) {
 $fromname = $object->personal;
 $fromaddress = $object->mailbox . "@" . $object->host;
}
Would give you two variables for the friendly from and the smtp from address
Thanks to www.natrak.net for help with this

An email without a subject line will not generate the 'subject' property.
Before using the 'subject' property you should test for it.
if (property_exists($header, 'subject')) echo $header->subject;