I Get This Notice: `Undefined index: searchterm` when i Search any Word

I'm sure my php code is fine. But when i use it in between html codes i cant receive the search elemet.

$search = mysql_real_escape_string(trim($_POST['searchterm']));

it fetches all records of my table.

<div id="spaces-main" class="pt-perspective">
<section class="page-section Portfolio-page page-section-57">
    <div class="large-7 columns p-l-0">
           <h2>
             Portfolio
           </h2>
    </div>

<div class="row portfolio-page">
              <div class="hide" id="project-info">
                 <div id="content">
                    <i class="fa-times fa back"></i>
                    <div class="row">
                       <div class="large-11 columns">
                          <h2 class="project-title">Super Design</h2>
                       </div>
                       <div class="large-6 columns">
                          <div class="project-images">
                             <div class="item-list">
                                <ul class="">
                                   <li>
                                      <a href="#">
                                      <img width="500" height="380" alt="13tndZF" class="attachment-project-thumb" src="assets/images/12.jpg">  
                                      </a>
                                   </li>
                                </ul>
                             </div>
                          </div>
                       </div>
                       <div class="large-6 columns project-content">
                          <?php 
           //The Problem Is Here!
                      $search = mysql_real_escape_string(trim($_POST['searchterm']));


                          $hostname='localhost';
                          $username='root';
                          $password='';

                          try {
                              $dbh = new PDO("mysql:host=$hostname;dbname=search",$username,$password);
                               $dbh->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
                              $dbh->exec("SET CHARACTER SET utf8");  //  return all sql requests as UTF-8

                              $sql = "SELECT * FROM a WHERE Name LIKE '%$search%' LIMIT 2";
                              $stmt = $dbh->query($sql);
                            foreach ($stmt as $row)
                                {
                                echo " Name : ".$row["Name"] ."<br/> Persian : ". $row["Persian"] ."<br/> Hindi :".$row["Hindi"]."<br/>";
                                }


                                $dbh = null;
                                }
                            catch(PDOException $e)
                                {
                                echo $e->getMessage();
                                }



                           ?>

                        </div>
                    </div>
                 </div>
              </div>

Could you please tell me what's wrong?