Generare una stringa casuale

/* tipo di caratteri
all = tutti
numbers = solo numeri
letters = solo lettere
lettersmax = solo lettere maiuscole
lettersmin = solo lettere minuscole
numberslettersmax = numeri e lettere maiuscole
numberslettersmin = numeri e lettere minuscole
*/

/**
 * Genera una stringa alfanumerica
 * @param  integer [$length = 10]             Lunghezza della stringa da generare
 * @param  string  [$characters_type = 'all'] tipo di caratteri
 * @return string  Stringa generata
 */
function genera_string($length = 10, $characters_type = 'all') {

    // Variabili
    $characters = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';;

    if ($characters_type == 'all') {
        $characters = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
    }
    elseif ($characters_type == 'numbers') {
        $characters = '0123456789';
    }
    elseif ($characters_type == 'letters') {
        $characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
    }
    elseif ($characters_type == 'lettersmax') {
        $characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    }
    elseif ($characters_type == 'lettersmin') {
        $characters = 'abcdefghijklmnopqrstuvwxyz';
    }
    elseif ($characters_type == 'numberslettersmax') {
        $characters = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    }
    elseif ($characters_type == 'numberslettersmin') {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
    }

    // Numero di caratteri
    $charactersLength = strlen($characters);

    // Creazione della stringa
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }

    // Stringa definitiva
    return $randomString;
}