PHP replacing special characters like à->a, è->e

Question

I have php document signup.php which save the content from form (in form.php document) to MySQL base. The problem arises when I want to reformat the input content. I want do

Answer 1

Wish I found this thread sooner. The function I made (that took me way too long) is below:

function CheckLetters($field){
    $letters = [
        0 => "a à á â ä æ ã å ā",
        1 => "c ç ć č",
        2 => "e é è ê ë ę ė ē",
        3 => "i ī į í ì ï î",
        4 => "l ł",
        5 => "n ñ ń",
        6 => "o ō ø œ õ ó ò ö ô",
        7 => "s ß ś š",
        8 => "u ū ú ù ü û",
        9 => "w ŵ",
        10 => "y ŷ ÿ",
        11 => "z ź ž ż",
    ];
    foreach ($letters as &$values){
        $newValue = substr($values, 0, 1);
        $values = substr($values, 2, strlen($values));
        $values = explode(" ", $values);
        foreach ($values as &$oldValue){
            while (strpos($field,$oldValue) !== false){
                $field = preg_replace("/" . $oldValue . '/', $newValue, $field, 1);
            }
        }
    }
    return $field;
}