How can I concatenate a string with variables in PHP?

Here is my simple code logic just for the sake of simplicity. I wanna concatenate string like: 10,11,12,13,14, but it's just adding all the values displaying wrong output as: ,100 it should display like: 10,11,12,13,14 like other programming languages do.

<?php 

    // list is a string variable
    $list="";

    //total number of seats in a Bus
    $setnum=10;

    for($i = 0; $i < 10; $i++) {
        //i trying to concatenate string like 10,11,12,13,14 
        $list="$list".$i+$setnum.', ';
    }

    echo $list; 

?>