PHP Unique Key Generator

/**
 * Generates a unique key
 * Less than one chance over 1.343646e+111 to bump into an existing key. If it happens, it just generates another key.
 * @param PDO $db        : the PDO instance
 * @param int $length    : the length of the generated key
 * @param string $table  : the table to check for the existing keys
 * @param string $column : the column in $table that contains the key
 * @return string        : the generated unique key
 */
function generateUniqId($db, $length, $table, $column) {
    $query = null;
    $key = '';
    $chars = array(
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'
    );

    do {
        while (strlen($key) < $length) {
            $key .= $chars[rand(0, count($chars))];
        }

        $query = $db->prepare('SELECT * FROM :tablename WHERE :keyname = :keyvalue');
        $query->execute(array(
            'tablename' => $table,
            'keyname' => $column,
            'keyvalue' => $key,
            'length' => $length
        ));
    } while ($query->rowCount() > 0);

    return $key;
}