Merge first column that has the same second column - php

Question

I have an INNER JOIN query that returns this result:
first column | second column
2 | 15
5 | 16
6 | 16
5 | 18
6 | 22
I want to output them in my page like this:
first column | second column
2 | 15
5,6 | 16
| 18
| 22
Total first column: 2
It will combine the first column that has the same second column. And value of the combined first column will be share between them.
And would also tell the total first column (which in the example is 2).
I'm still trying with this code:
if($stmt = $con->prepare("SELECT firsttb.firstcolumn, secondtb.secondcolumn FROM firsttb INNER JOIN secondtb ON firsttb.connectid = secondtb.connectid WHERE firsttb.getid = ? ORDER BY secondtb.secondcolumn")){
$stmt->bind_param("i",$_GET["getid"]);
$stmt->execute();
$stmt->bind_result($firstcolumn,$secondcolumn);
$lastid = 0;
$total = 0;
$combine = "";
while($stmt->fetch()){
if($lastid == $secondcolumn){
$combine = $combine."".$firstcolumn;
$lastid = $secondcolumn;
}
else {
$totalrow = $totalrow + 1;
$lastid = $secondcolumn;
$combine = $secondcolumn;
echo $combine." | ".$secondcolumn."<br>";
}
}
$stmt->close();
echo "Total first column: ".$totalrow;
}
But only get this output:
first column | second column
2 | 15
5 | 16
5 | 18
6 | 22
Total first column: 4
It is okay to scrap my code and create it with your own. Just want to look for the right idea.