Checkbox with multiple values and same name returning a NULL value

here is my form

  man utd group<input type='checkbox' name='groups_hash_value[]' value='2147483647'><span style='font-size:50%;'>SMS and Email</span>arsenal fixtures<input type='checkbox' name='groups_hash_value[]' value='9'><br><br>

  Firstname:<br>
  <input type="text" name="firstname" value="d"><br>
  Surname:<br>
  <input type="text" name="surname" value="burke"><br>
  Mobile Number:<br>
  <input type="text" name="telephone" value="080"><br>
  Email:<br>
  <input type="text" name="email" value="dara@gmail.com"><br>

  <p style="font-size:10px;">
  Your info will not be sold, traded or given away <br>
  Brought to you by the <a href="http://www.{website_name}" >web marketing</a> company <a href="http://www.">{website_name}</a><br>
  </p>
  <input type="submit" value="Submit">
</form> 

this gives me a null value when i check the two checkboxes and then i var_dump($_POST)