PHP未定义集

在第23行收到通知注意：未定义偏移：5？在23号线上？对不起，我想不通，有人能帮我吗？顶部线条和底部线条只是不在中间。THanks显示添加了新代码请

$mystring = "level"; 
echo "String: " . $mystring;
$str=$mystring;`enter code here`
$mystring = trim( preg_replace( "/[^0-9a-z]+/i", " ", $mystring) );
$myArray = array();
$myArray = str_split($mystring); 
$len = sizeof($myArray); 
$newString = "";
for ($i = $len; $i >= 0; $i--) {
  $newString.=$myArray[$i];
}
echo "<br>";
if ($mystring == $newString) {
  if ($str=$newString) {
    echo "Output: " . $mystring . " is a perfect palindrome";
  } else {
    echo "Output: " . $mystring . " is a Standard palindrome";
  }
} else {
  echo "Output: " . $mystring . " is not a palindrome";
}