update datetime with null value in mysql by php

I’m using this code to update my datatime field to NULL, php version 7.3.7

    if($_POST['value']=='0000-00-00 00:00:00'){
      $timestamp=NULL;
    }else {
      $dateTime = $_POST['value'];
      $timestamp = date('Y-m-d H:i', strtotime($_POST['value']));
   }
$query="update forms set $_POST[limitInputField]='$timestamp' where  formid='$_POST[formId]'";
$result=$dbCnn->query($query)or die($dbCnn->error);

it gives error when $timestamp is null:

Incorrect datetime value: ” for column farsifor_m.forms.enddate at row 1

But when I quote “NULL” and remove quotations around $timestamp in the query, if $timestamp is null it works properly but if $timestamp value is not null it gives error.

   if($_POST['value']=='0000-00-00 00:00:00'){
     $timestamp="NULL";
   }else {
     $dateTime = $_POST['value'];
     $timestamp = date('Y-m-d H:i', strtotime($_POST['value']));
   }
  $query="update forms set $_POST[limitInputField]=$timestamp where       formid='$_POST[formId]'";
    $result=$dbCnn->query($query)or die($dbCnn->error);

Answer

Learn to use prepared statements; and do not inject post variables to “build” the query. Having said that:

You can use NULLIF for convenience. In the following example the specific value 0000-00-00 00:00:00 will be converted to NULL:

UPDATE forms SET col = NULLIF(:timestamp, '0000-00-00 00:00:00') WHERE formid = :formid

Or you can simply:

UPDATE forms SET col = :timestamp WHERE formid = :formid

And use PHP to pass a variable containing string or null.