$email='john.doe@domain.tld';
list($first, $last) = explode('@', $email);
$localPart = strlen($first) < 3 ? str_repeat('*', strlen($first)) : substr($email, 0, 1).str_repeat('*', strlen($first) - 2).substr($first, -1);
$domain = strlen(substr($last, 0, strrpos($last, '.'))) < 3 ? str_repeat('*', strlen(substr($last, 0, strrpos($last, '.')))) : substr($last, 0, 1).str_repeat('*', strrpos($last, '.') - 2).substr($last,strrpos($last, '.') - 1, 1);
$tld = substr($last, strrpos($last, '.'));
echo $localPart.'@'.$domain.$tld; // j******e@d****n.tld