Need to Auto generate key in Alphanumeric

$x = 'AA998';
for($i = 0; $i < 10; $i++) {
    echo $x++,'<br />';
}

$x = '000AY';
for($i = 0; $i < 10; $i++) {
    echo $x++,'<br />';
}

$x = 'ZZ998';
for($i = 0; $i < 10; $i++) {
    $x++;
    if (strlen($x) > 5)
        $x = '000AA';
    echo $x,'<br />';
}

$charset = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
$str = '';
$length = strlen($charset);
$count = 12;
while ($count--) {
    $str .= $charset[mt_rand(0, $length-1)];
}
echo $str;

digits="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

def NthStr(n):
    str=digits[n%len(digits)];
    while n>=len(digits):
        n=(n/len(digits) - 1)
        str=digits[n%len(digits)]+str
    return str

public string Next(string cur)
{
    int num = int.Parse(cur.Substring(1));
    char chr = cur[0];
    num++;
    if (num > 9999)
    {
        num = 0;
        chr++;
    }
    return chr + num.ToString().PadLeft(4, '0');
}

select Max(id) from voiceno