implode an array into a comma separated string from mysql query

For the last 1 1/2 days I've been trying to store 16 row id's into a string and separate each id with a comma. The array I am getting is from MySQL. The error I am getting is

implode() function:passed invalid arguments

$str=array();
$string="";
while($row = mysql_fetch_row($result)) 
{
    $user_id=$row;
    $str=$user_id;
    foreach($str as $p=>$v){
        comma($v);
    }
}

function comma($v){
    $string= implode(",",$v); echo $string;
}