mysql PHP auto increment column

How do i add an auto incrementing column called ID to these statements:

$sql = "CREATE TABLE IF NOT EXISTS ".$username." (uid varchar(255), tweet varchar(255))";
$alter = "ALTER TABLE ".$username." ADD UNIQUE (uid)";
开发者_StackOverflow

Thanks

I would do

"CREATE TABLE IF NOT EXISTS ".$username." (id MEDIUMINT AUTO_INCREMENT PRIMARY KEY, uid varchar(255) UNIQUE, tweet varchar(255))";

$sql = "CREATE TABLE IF NOT EXISTS ".$username." (ID int not null auto_increment primary key, uid varchar(255), tweet varchar(255))";