Named parameter in update where clause (pdo object) - php

Question

I try to translate this query to my PDO object from this thread:
UPDATE table_name
SET col1 = <<new value>>,
col2 = <<new values>>,
last_modified_timestamp = <<new timestamp>>
WHERE primary_key = <<key column>>
AND last_modified_timestamp = <<last modified timestamp you originally queried>>
So i have a "modified" field in the mysql table and fetch the data (SELECT modified AS last_modified) to pre-fill in a hidden field in my form and post the value to the object:
$position->readOne();
$position->last_modified = $_POST['last_modified'];
<input name='last_modified' value='{$position->last_modified}'>
My object update query looks like:
UPDATE positions
SET
... some values ...
WHERE id=:id
AND modified=:last_modified
$stmt->bindParam(":last_modified", $this->last_modified);
If I check the posted variables, everything looks fine but the update query ignores my second where clause completely and override the modified field after post the form.
Sure a beginner issue but I can´t find it.
Thanks
EDIT:
Select query
public function readOne(){
$query = "SELECT
p.position,
p.modified,
p.modified AS last_modified
FROM positions p
WHERE id = ?
LIMIT 0,1";
$stmt = $this->conn->prepare( $query );
$stmt->bindParam(1, $this->id);
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_ASSOC);
$this->position = $row['position'];
$this->modified = $row['modified'];
$this->last_modified = $row['last_modified'];
}
Update query
public function updatePosition(){
$this->getTimestamp();
$query = "UPDATE positions
SET
position=:position,
modified=:modified,
WHERE id=:id
AND modified=:last_modified";
$stmt = $this->conn->prepare($query);
$this->position=htmlspecialchars(strip_tags($this->position));
$stmt->bindParam(":id", $this->id);
$stmt->bindParam(":position", $this->position);
$stmt->bindParam(":modified", $this->timestamp);
$stmt->bindParam(":last_modified", $this->last_modified);
if($stmt->execute()){
print_r($this->last_modified);
return true;
}
print_r($stmt->errorInfo());
return false;
}
public function getTimestamp(){
date_default_timezone_set('Europe/Berlin');
$this->timestamp = date('Y-m-d H:i:s');
}

Answer 1

I figured it out. The update query and everything else worked fine but the problem was the notification handling, so the function updatePosition() told me everything was updated but it worked as it should and it did not update anything if the value was not the same.
This fixed it:
if($stmt->execute()){
$affected_rows = $stmt->rowCount();
if ($affected_rows == 1) {
return true;
}
This helped me to understand how it works ... PDOStatement::execute() returns true but the data is not updated