php - get value of checkbox submit to a variable -

i'm trying work out how value of checkbox submit variable id sql db.

below current code:

-- index.php

<?php      $sql = "select * `file`";        if (!$result = $db->query($sql)){           die('there error running query [' . $db->error . ']');           }     ?>      <form method="post">      <a href="includes/aed/file_add.php" data-toggle="modal" data-target="#new"><button class="btn btn-success">new</button></a>     <a href="includes/aed/file_edit.php" data-toggle="modal" data-target="#edit"><button type="submit" class="btn btn-warning">edit</button></a>     <a href="includes/aed/file_delete.php" data-toggle="modal" data-target="#delete"><button class="btn btn-danger">delete</button></a>      <table class="table table-condensed">        <thead>         <th>#</th>         <th>name</th>         <th>category</th>         <th>date</th>       </thead>        <tbody>          <?php  while($row = $result->fetch_assoc()){ ?>          <td><input type="checkbox" name="id_grab" value="<?php echo $row['file_id']; ?>"></td>         <td><a href="simple/files/<?php echo $row['file_location']; ?>"><?php echo $row['file_name']; ?></a></td>         <td><?php echo $row['file_category']; ?></td>         <td><?php echo date('d-m-y', strtotime($row['file_date'])); ?></td>        </tbody>      <?php } ?>     </table>      </form> 

-- file_edit.php

<?php include('../db_connect.php') ;  $id=isset($_post['id_grab']);      $sql = "select * `file` file_id = $id";        if (!$result = $db->query($sql)){           die('there error running query [' . $db->error . ']');           }          $row = $result->fetch_assoc(); ?>    <div class="modal-header">     <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;       </button>     <h4 class="modal-title" id="mymodallabel">edit invoice / receipt</h4>   </div>   <div class="modal-body">      <form action="../includes/file_update.php" method="post" role="form" enctype="multipart/form-data">          <div class="form-group">             <label for="file_name">name</label>             <input type="text" name="file_name" class="form-control" id="file_name" value="<?php echo $row['file_name']; ?>">         </div>          <div class="form-group">             <lable for="file_category">category</label>             <input type="text" name="file_category" class="form-control" id="file_category" placeholder="financial">         </div>          <div class="form-group">             <label for="file_tag">tag</label>             <input type="text" name="file_tag" class="form-control" id="file_tag" placeholder="statement">         </div>          <div class="form-group">             <label for="file_description">description</label>             <input type="text" name="file_description" class="form-control" id="file_description">         </div>          <div class="form-group">             <label for="file_date">date</label>             <input type="date" name="file_date" class="form-control" id="file_date">         </div>          <div class="form-group">             <label for="file_location">file</label>             <input type="file" class="form-control" name="file_location" value="file">             <p class="help-block">select appropriate file upload.</p>         </div>      <button type="button" class="btn btn-info" data-dismiss="modal">cancel</button>     <button type="submit" class="btn btn-success">upload</button>       </form> </div> 

i can't work out, spent hours looking online... concern @ it's using bootstraps modal wasn't sure if able grat post data... if enter id manually works...

any assistance appreciated.

change

$id=isset($_post['id_grab']); 

to

$id=isset($_post['id_grab'])?$_post['id_grab']:''; //if id_grab dont have value $id value '' 

isset() returns true if var exists , has value other null, false otherwise.