why undefined index error in php code example

Example 1: php undefined index

// Your array index has no value or is not referencing anything,
// The easiest way to overcome this is to simply check whether
// it has been defined
if (isset($arr[$i])) {
	// Do something
}

Example 2: php Undefined index: error

// Before using $_POST['value']    if (isset($_POST['value']))    {              // Instructions if $_POST['value'] exist    }