Php foreach loop concatenation

I am working on a script to loop over what check boxes a user clicks on, depending on which ones were clicked that will result in a "dynamic" query. I am trying to build a where statement with a array to execute in pdo.

The problem I am having is that within the foreach loop I am trying to use

.=

to concatenate a string to build my query. However when a user clicks on the same group of checkboxes like

'why_and'

only one $where will be created so when I print out my query with a var_dump I get this :

SELECT student.anum, 
       student.first, 
       student.last, 
       student.email, 
       session.aidyear, 
       reasons.reason, 
       COUNT(session.anum) as Total, 
       MIN(DATE_FORMAT(session.signintime, '%b %d, %Y - %l:%i %p')) as 'First',      
       MAX(DATE_FORMAT(session.signintime, '%b %d, %Y - %l:%i %p')) as 'Last' 
FROM  
      student INNER JOIN session 
           ON session.anum = student.anum 
      INNER JOIN session_status 
           ON session_status.status = session.status 
      INNER JOIN reasons 
           ON reason_id = session.why 
WHERE 1 AND why = :reason 
GROUP BY session.anum

and then I var_dump the placeholder array and get this :

[":reason"]=> string(9) "4 , 5 , 6" }

Notice how I have three values in the :reason array and only 1 :reason in the query. Is there anything I am missing?

This is a example with my thought process (please take a look at the comments) :

   $placeholder = array();
    $where = "";

if($and !== "") // This is the "super array" if you will 
{
    if(array_key_exists('why_and', $and)) // Now I check for each specific thing a user can search for in the array / database 
    {
       foreach($and as $key => $value) // Take the $value of the array index why_and
       {
           $where .= " AND why = :reason "; // Create a where variable with a named placer holder for each value that exists in the array 
           $placeholder[':reason'] .= rtrim($value, ' ,'); // Then add to the $placeholder array so I can I add it to the PDO execute as an array
       }
    }
    if(array_key_exists('status_and', $and))
    {
       foreach($and as $key => $value)
       {
           $where .= " AND status = :status ";
           $placeholder[':status'] .= rtrim($value, ' ,');
       }
    }
}
$finSQL = $sql . ' WHERE 1 ' . $where; // COncate the final results
$dynamic = $this-> db-> conn_id-> prepare($finSQL); // prepare
$dynamic-> execute($placeholder); // Use the placeholder array and pass that to the execute as a [key => value pair as shown in the manual example number 2][1]

Edit 1 @Your Common Sense

Just to clear things up THIS WORKS but only for one value in each check box category, why_and, status_and but if I try to do more then one value from the same category then it does not work.