What do you mean by normalization? Just take the characters as they come in, but put your scripts, connection and html in correct encoding.
UTF-8 suggested, explanation: UTF-8 vs. Unicode
If you only want ASCII chars, you can test this by something like ord($char) < 255
.
For conversion look into these functions:
http://php.net/mb_convert_encoding
A call similiar to
$str = iconv('UTF-8', 'ASCII//TRANSLIT', $str);
would do the trick.
Another preg_replace way: Convert special characters to normal characters using PHP, like ã, é, ç to a, e, c