javascript - trying to post an array in a db without success -

so have form allows me create new user:

<?php include 'header.php';  require_once '../../db_con.php';   try{     $results = $dbh->query("select * cat_list");  }catch(exception $e) {     echo $e->getmessage();     die(); }  $category = $results->fetchall(pdo::fetch_assoc);  ?>      <form action="actions/add_emp.php" method="post">        <input type="text" name="user" placeholder="username" required="required"  /><br/>       <input type="text" name="pass" type="password" placeholder="password"/></label><br/>        <input type="hidden" name="action" value="id"/>          <input type="text" name="firstname" id="name" required="required" placeholder="firstname"/><br />         <input type="text" name="lastname" id="email" required="required" placeholder="lastname"/><br/>         <input type="email" name="email" id="city" required="required" placeholder="email address"/><br/>         <input type="text" name="extension" id="extension" required="required" placeholder="extension number"/><br/>         <select name="title">             <option  selected disabled>please select job title...</option>              <option disabled></option>             <option disabled>helpesk:</option>             <option value="technical support advisor">technical support advisor</option>             <option value="deputy team leade">deputy team leader</option>             <option value="team leader">team leader</option>             <option value="incident resolution advisor">incident resolution advisor</option>              <option disabled></option>             <option disabled>call centre:</option>             <option value="technical support advisor">technical support advisor</option>             <option value="">deputy team leader</option>             <option value="">team leader</option>              <option disabled></option>         </select>   <div id="checkboxlist" >     <?php         foreach($category $cat){      ?>      <input type="checkbox" value="<?php echo $cat["cat_id"]; ?>" name="cat_no[]" id="box1"> <?php echo $cat["cat_title"]; ?></a><br>      <?php     }     ?> </div>           <input type="submit" value="add user" name="submit"/><br />     </form> 

my action script looks this:

<? session_start(); session_regenerate_id(); /******************************************************************* **  action script post new category doc_list table  ** *******************************************************************/       if(isset($_post["action"])){     if(isset($_post['submit'])){     include_once'../../config.php';      $dbh = new pdo("mysql:host=$hostname;dbname=dashboardr",$username,$password);     $dbh->setattribute(pdo::attr_errmode, pdo::errmode_exception);       if(isset($_post['user']) && isset($_post['pass'])){         $password=$_post['pass'];         $sql=$dbh->prepare("select count(*) `user_login` `username`=?");         $sql->execute(array($_post['user']));      if($sql->fetchcolumn()!=0){      die("user exists");       }else{         function rand_string($length) {         $str="";         $chars = "subinsblogabcdefghijklmanopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789";         $size = strlen($chars);         for($i = 0;$i < $length;$i++) {         $str .= $chars[rand(0,$size-1)];     }      return $str;      }         $p_salt = rand_string(20);          $site_salt="subinsblogsalt";          $salted_hash = hash('sha256', $password.$site_salt.$p_salt);          $sql=$dbh->prepare("insert `user_login` (`id`, `username`, `password`, `psalt`, `firstname`, `lastname`, `email`, `extension`)                  values (null, ?, ?, ?, ?, ?, ?, ?)");         $sql->execute(         array($_post['user'], $salted_hash, $p_salt, $_post["firstname"], $_post["lastname"], $_post["email"], $_post["extension"]));       }    $userid = $dbh->lastinsertid();  /*********************************************************************** **  inserts array depending on checked within form  ** ***********************************************************************/   $sql = "insert `user_cat_link_table`(`userid`, `cat_id`) values"; $values = ""; $params = [];  foreach($_post["cat_no"] $cat)  var_dump($_post);   {      $values.= "(?, ?), ";     $params[] = $cat; // correct here     $params[] = $userid; }      $values = substr($values, 0, -2);     $sql.= $values;     $query = $dbh->prepare($sql);     $query->execute($params);             if ($dbh->query($sql)) {      }else{}      $dbh = null;     }      {     header ('location: ../list_emp.php?success=1');     }      }  } 

now when var_dump($_post); gives me results expecting not posting join table:

array(9) { ["user"]=> string(19) "2342rrwersfsdfdffdf" ["pass"]=> string(3) "424" ["action"]=> string(2) "id" ["firstname"]=> string(5) "23423" ["lastname"]=> string(5) "23424" ["email"]=> string(26) "daniel.barrett@ricoh.co.uk" ["extension"]=> string(6) "312321" ["cat_no"]=> array(1) { [0]=> string(2) "61" } ["submit"]=> string(8) "add user" } 

i taking id of user created using $userid = $dbh->lastinsertid(); , linking category id selcted , post koin table takes users id , category id selected.