Generate Alpha Numeric number

Some times you want to generate Alpha Numeric , Numeric or Alphabetic digit for password or some kind of number. It can be 2 to 16 digit. For these problem i have a solutions:

function generateId($length, $type = 'ALPHANUMERIC') {

if($type == 'ALPHANUMERIC')
$chars = "12345CDEFGHI67890NOPQRSTU01492VWXYZ43210XYZFGH98765ABCDEF83574GHIJKLM09876QRSTUV54321";
else if($type == 'NUMERIC')
$chars = "1234567890987654321678905432101928374651029384756162738495061728394055432167890";
$id = "";
while (strlen($id)<$length) {
$id .= $chars{mt_rand(0,strlen($chars))};
}

return $id;

}

generateId(12,  'ALPHANUMERIC');