Removing duplicate values within a comma separated list in PHP

How can I remove duplicate values within a comma separated list in PHP?

I have 2 versions of a customer questionnaire - one is more detailed than the other - and in one they are asked the items they like and in the other they are asked to specify the items they like in ranked order. In this particular example, one of the columns ('items_like') may contain several values in a comma separated list. There are two other scenarios where each of the columns may contain several values in a comma separated list.

In order to display a list of all the items a customer likes, I am concatenating the 4 columns when the information is fetched from the db. How can I remove any duplicate values from the combined comma separated list in? I'd prefer doing this in PHP than Javascript

try {  
$stmt = $conn->prepare("SELECT * FROM customer_info WHERE user_id = :user_id"); 
$stmt->bindValue(':user_id', $user_id); 
$stmt->execute();
}catch(PDOException $e) {echo $e->getMessage();}
$row = $stmt->fetch();
$search = array('_', ',', 'Null');
$replace = array(' ', ', ', '');
$rows = str_replace($search, $replace, $row);

$merged_likes = $rows['items_like']. ",  " . $rows['first_like']. ",  " .     $rows['second_like']. ",  " .  $rows['third_like'];
echo $merged_likes;

When dealing with lists of things, it's easiest to use arrays. Comma-separated strings work alright for displaying things, but they're no fun to work with in code. With that in mind, I would recommend not even touching a comma until you finally display the data out to the screen.

Unfortunately, it sounds like you already have comma separated strings stored in your database. If this is something you can change, you may want to look into an alternative database structure using multiple tables with a one-to-many relationship between users and things they like. There's a lot that could be said about this, but it's not the subject of your question. The basic concept is called database normalization, as Havelock has already mentioned.

In the event the database structure cannot be changed and you must work the comma-separated lists, simply undo the comma separation and get them back into arrays in PHP. To do this, you'd use the explode() function:

$teststring = '1,2,3';
$test = explode(',', $asdfstring); // array(1, 2, 3)

Once you have arrays, PHP has some handy features for sorting, removing duplicates, etc. The one you're probably most interested in here is array_unique().

To put it back into a comma-separated list for display, simply use implode(), which does the opposite of explode() by taking an array and joining it with the separator of your choice (e.g., comma).

Now since you have multiple columns you're dealing with here, and you only want one list, you're going to have to build the array in multiple steps. If every one of these columns can contain multiple values, you can do the explosion thing on all of them:

$items_like = explode(',', $rows['items_like']);
$first_like = explode(',', $rows['first_like']);
$second_like = explode(',', $rows['second_like']);
$third_like = explode(',', $rows['third_like']);
$merged = array_merge($items_like, $first_like, $second_like, $third_like);

Or if first, second, and third will only contain one item, you could do this:

$items_like = explode(',', $rows['items_like']);
$other_likes = array($rows['first_like'], $rows['second_like'], $rows['third_like']);
$merged = array_merge($items_like, $other_likes);

Now remove duplicates: