MySQL SELECT w/ two AS id field names using php while mysql_fetch_array() to identify them uniquely

$sql = "
(SELECT
a.id as AID,
a.Title,
a.Category,
c.id as CID,
c.cateName FROM ads_list
AS a LEFT JOIN ads_cate
AS c ON c.id=a.Category WHERE a.Category = $CatID)";

SELECT
  a.id AS aid,
  a.Title
  a.Category
  c.id AS cid,
  c.cateName
FROM ads_list a 
   LEFT JOIN ads_cate c ON c.id=a.Category WHERE a.Category = $CatID)

Have fun

You have two options:

1- give each id an alias:

SELECT
a.id AS aid,
a.Title,
a.Category,
c.id AS cid,
...

and address them as $row[ 'aid' ] and $row[ 'cid' ].

2- use mysql_fetch_row() instead of mysql_fetch_assoc() and address them as $row[0] and $row[3].

you can change your query to something like:

SELECT
a.id as a_id,
a.Title,
a.Category,
c.id as c_id,
c.cateName FROM ads_list
AS a LEFT JOIN ads_cate
AS c ON c.id=a.Category WHERE a.Category = $CatID

that way there is no ambiguity in column names and you can access it as $row['a_id'] or $row['c_id']

Can't you just remove c.id from query? You don't need that within SELECT statement if you need it in your query:

$sql = "SELECT a.id, a.Title, a.Category, c.cateName 
FROM ads_list AS a 
LEFT JOIN ads_cate AS c ON c.id=a.Category 
WHERE a.Category = $CatID"; 

$result = mysql_query($sql) or die(mysql_error());

while ($row = mysql_fetch_assoc($result)) { echo $row['id'] // equals c.id }

If you need it though, you can also use PDO and value binding. I do not recommend using mysql_* functions. PDO is just better and easier to read:

  $query = $pdo->query('SELECT a.id, a.Title, a.Category, c.id, c.cateName 
                               FROM ads_list AS a 
                               LEFT JOIN ads_cate AS c ON c.id=a.Category 
                               WHERE a.Category = $CatID');

  $query->bindColumn(1, $id_a); //numerical index goes from 1
  $query->bindColumn(4, $id_b);
  $query->bindColumn('Title', $title); //you can also index using column name if it's possible

  while ($query->fetch(PDO::FETCH_BOUND)) //fetch returns true if there are any records to fetch
  {
    echo $id_a;
    echo $id_b;
    echo $title;
  }

Downside of this solution (and mysql_fetch_row) is that any change within the query can result in change of indexes. And even if now it's not the problem, believe me - I've seen a query with 30 returned columns and mysql_fetch_row applied. It was a nightmare. Impossible to find out which column in database is represented by certain variable.