<?PHP
$headers=imap_fetchheader($imap, $msgid);
preg_match_all('/([^: ]+): (.+?(?:\r\n\s(?:.+?))*)\r\n/m', $headers, $matches);
?>
Works quite well for splitting headers,
$matches will contain 3 arrays:
$matches[0] are the full-lines (To: Jille@devnull.quis.cx\r\n)
$matches[1] will be the header (To)
$matches[2] will be the value (Jille@devnull.quis.cx)
In multi-line values the 'multilining is not stripped!'
this could be achieved with something like:
<?PHP
preg_replace('/\r\n\s+/m', '', $matches[2]);
?>