Mysql Left join index

I used the following query

select a.*,b.* from firsttable a left join secondtable b on a.id=b.item_id ORDER BY a.id DESC LIMIT 0,10

To display items from two tables, where the id of the first table is the item_id of the second. My question is , when I try to display this in php , if I want to display a.id i can try with:

while($row=$go->fetch_assoc()){
echo $row['id'];
}

or

while($row=$go->fetch_assoc()){
echo $row['a.id'];
}

since both tables have id,on the first example displays only if there are matching rows from first and second table and displays the id of second table, I want the id of first

and on the second it says undefined index. Can you explain why is this please?

Edit: Adding tables for example

id
info
username

id
item.id
username

Answer

Both tables have a column that has the same name, so, when using select *, it is ambiguous to which column id relates to.

The only way to remove the ambiguity is to explicitly list all the columns you want to select, using aliases for homonyms:

select 
    a.id, 
    b.id b_id, -- alias for b.id
    b.item_id
    -- more columns here as needed
from firsttable a 
left join secondtable b on a.id=b.item_id 
order by a.id desc 
limit 0,10

This is one of the many reasons why select * is generally considered a bad practice in SQL.

Recommend reading: What is the reason not to use select *?