Fixing badly-encoded UTF-8 characters in MySQL database

fix.php

<?php
// Go home PHP, you're drunk.

// No charset here
$old_db = new PDO('mysql:host=localhost;dbname=database', 'username', 'password');
$old_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
// Charset here
$new_db = new PDO('mysql:charset=utf8mb4;host=localhost;dbname=database2', 'username', 'password');
$new_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

$result = $old_db->query('SELECT * FROM items');
$row = $result->fetch(PDO::FETCH_ASSOC);
$columns = array_keys($row);
// [foo, bar] => [':foo', ':bar'] for query params
$placeholders = array_map(function($column) { return ':' . $column; }, $columns);

// INSERT INTO items (foo, bar) VALUES (:foo, :bar)
$insert = $new_db->prepare('INSERT INTO items (' . implode(', ', $columns) . ') VALUES (' . implode(', ', $placeholders) . ')');

do {
        $insert->execute($row);
        echo '.';
} while ($row = $result->fetch(PDO::FETCH_ASSOC));

echo "\nDone";