i need to insert array and an integer to my users_skills table

i have users_skills and there are Id as primary key , U_Id which is foreign key to user table on Id , S_Id foreign key to skill table on Id i need to insert to that table an array of skills and the user Id and here,s my php code

 <?php

  include('database.php');
  session_start();

  if(isset($_POST['AddSkills']))
  {
      $uid=$_SESSION['id'];
      $selected= $_POST['skill'];
      foreach ($selected as $s)
     {
        $sql = "INSERT INTO users_skills (U_Id,S_Id) " 
        . "VALUES ('$uid','$s')";
      }

     if ( $mysqli->query($sql) ){

       header("location:gallery.php"); 
         }

      else {

      header("location: r.php");
          }
      }
          ?>