An alternative to the mailparse_rfc822_parse_addresses() function is Mail_RFC822::parseAddressList() from Pear:
http://pear.php.net/manual/en/package.mail.mail.php
It parses the string and returns a structured tree of data. Returns a pear_error object if the string is not valid.
Example:
require_once "PEAR.php";
require_once "Mail/RFC822.php";
$addr= "Hi <hi@world.org>";
$res= Mail_RFC822::parseAddressList($addr);
if (PEAR::isError($res)) die("NOT VALID: " . $res->getMessage() . "\n");
echo "OK. Data:\n";
print_r($res);