rehash.php

<?php
$userEmail = 'test@test.com';
$userPassword = '123456';

// Using Doctrine for example
$user = $repository->findByEmail($userEmail);

if(password_verify($userPassword, $user->getPassword())){
    $options = ['cost' => 10];
    if(password_needs_rehash($user->getPassword(), PASSWORD_DEFAULT, $options)){
        $hash = password_hash($userPassword, PASSWORD_DEFAULT, $options);
        // save $hash as a NEW password on database
    }
    // DO LOGIN
}