How can I design this job stable and optimised

Hi to everyone. I need some advices.

I need to build an application with PHP and I'm at the next part:

I have an array with data and I need to check if the ID from this array is already inserted in my database:
- If it's already inserted in DB, I need to get the unique ID of row
- If it is not inserted in DB, I need to insert all data of array and get the new unique ID of inserted row.

I need to do this job optimized as possible! The table with content where this array will be inserted may have +500.000 rows and I must do this job for 20 arrays, not one (these arrays are data got from a feed source)

So, using a foreach loop my pseudocode will be something like:

foreach($feed_array as $feed_id){
-check if $feed_id already is in DB
- TRUE: get the ID of the row where $feed_id is in
- FALSE: insert into db some data (included $feed_id);
- get the ID of the row where $feed_id is in
} end foreach loop;

So this is what I want to do optimized as possible.

Thanks you.

 

//Assumes the rowid is the primary key of the table

foreach ($idarray as $id){
$result = mysql_query("SELECT * from feeds WHERE feedid=$id");
if (mysql_num_rows ($result ) > 0){
$row = mysql_fetch_assoc($result);
$rowid = $row['rowid'];
}else{
mysql_query("INSERT into table() values()");
$rowid = $row['rowid'];
$rowid = mysql_insert_id();
}
}