Password changing code error - php

Question

I am using password_hash function, it work's well in registration form and login form but doesn't work during change password form, it gives me error message incorrect old password may be my code has gone wrong or may be because password_hash generates different set of characters each time even with the same input, if it is so what method is used to update password. the same code works using md5.
if(isset($_POST['senddata'])){
$old_password = $_POST['oldpassword'];
$new_password = $_POST['newpassword'];
$repeat_password = $_POST['newpassword2'];
$query = $db->prepare("SELECT * FROM users WHERE username=:username");
$query->execute(array(':username'=>$username));
$row = $query->fetch(PDO::FETCH_ASSOC);
$db_password=$row['password'];
// hash old password before match
$old_password = password_hash($old_password, PASSWORD_DEFAULT);
// check if old password equals db_password
if ($old_password==$db_password) {
// continue changing users password
if ($new_password==$repeat_password) {
// hash the new password
$new_password=password_hash($new_password, PASSWORD_DEFAULT);
$repeat_password=password_hash($repeat_password, PASSWORD_DEFAULT);
// update password
$password_update_query=$db->prepare("UPDATE userss SET password=:password, password2=:password2 WHERE username=:username");
$password_update_query->execute(array(':password'=>$new_password,':password2'=>$repeat_password2,':username'=>$username));
echo "Your Password Updated";
}
} else {
echo "Old password is incorrect";
}
}

Answer 1

You need to use password_verify($password, $hash); for verifying that passwords are equal
When you hash it again you get a other result because it generates a new salt, which then result in an other hash.
Something like:
$old_password = $_POST['oldpassword'];
$db_password = $row['password']; // which should be already hashed
if (password_verify($old_password, $db_password) {