PHP PDO fetch data from Array

Question

This Code Works Perfectly For Mysqli

$sql = "SELECT * FROM 'mem_tree' ";
$res = mysqli_query($conn, $sql) or die("database error:".mysqli_error($conn));
while($row = mysqli_fetch_assoc($res)){ 
  $data[] = $row;
}
$itemsByReference = array();

but When i'm trying to do with PDO it Doesn't Work

$sql = "SELECT * FROM 'mem_tree' ";
$stmt = $dbh->prepare($sql);
$stmt->execute(); 
while($row = $stmt->fetchAll(PDO::FETCH_ASSOC)){ 
   $data[] = $row;
}
$itemsByReference = array();

Answer 1

Here i found Solution of it

  $sql = "SELECT * FROM mem_tree";
  $stmt = $dbh->prepare($sql);
  $stmt->execute();
  $final_arr = array();
  $data = array();
  $final_arr = $stmt->fetchAll(PDO::FETCH_ASSOC);
  $data = $final_arr;
  $itemsByReference = array();