$row will have an array. You can explode only strings, not arrays. And, as JRM mentioned, I don't recommend the use of mysql_fetch_row. Since this returns an numeric index array, you will have to use $row, $row etc.
Say, after sometime, you change the database structure, by adding/removing a field [first field for example], then that is going to be a big problem as $row will now correspond to the newly added field!
Always use mysql_fetch_array or mysql_fetch_assoc.