function extract_emails($str){
    // This regular expression extracts all emails from a string:
    $regexp = '/([a-z0-9_\.\-])+\@(([a-z0-9\-])+\.)+([a-z0-9]{2,4})+/i';
    preg_match_all($regexp, $str, $m);

    return isset($m[0]) ? $m[0] : array();
}

$test_string = 'This is a test string...

        test1@example.org

        Test different formats:
        test2@example.org;
        <a href="test3@example.org">foobar</a>
        <test4@example.org>

        strange formats:
        test5@example.org
        test6[at]example.org
        test7@example.net.org.com
        test8@ example.org
        test9@!foo!.org

        foobar
';

print_r(extract_emails($test_string));