PHP input text reading without getting error

You can use any of the following processes

if (isset($_POST['submit'])) { ... }

<input type="text" name="search_id" id="search_id"><input type="submit" name="submit" value="Submit" />

$search_id = isset($_POST['search_id'])?$_POST['search_id']:0;

another way is

$search_id="0";
 if(isset($_POST['sea'])){
 $search_id=$_POST['search_id'];
 if($search_id==""){
 $search_id="0";
 }
 }