Hot to fix this warning? [duplicate]

im new in PHP.
I want to make my first mini project “Todo APP” and i have a problem with $_POST array.

Error:
Warning: Undefined array key “task” in C:xampphtdocsTODOAPPindex.php on line 24

My code:

$task = $_POST["task"];
$error ="";
$todo = array ();

if(isset($_POST["submit"])){
  
if(empty($_POST["task"])){
    $error="najpierw wypełnij pole";    
}
else{
    array_unshift($todo, $_POST["task"]);
    $sql= "INSERT INTO `todo` (`id`, `task`) VALUES ('0', '$task')";
    $rs = mysqli_query($linkDB, $sql);
}
}

How can i fix this warning ?

I dont know how to fix it, i tried to change if with $task=$_POST["task"] in side but i think is bad form for code.