<?php
ini_set('max_execution_time', '0');
ini_set("max_input_time", '0');
set_time_limit(0);
error_reporting(E_ALL);

define('DBHOST', 'localhost');
define('DBUSER', 'user');
define('DBPASS', 'pass');
define('DBNAME', 'database');
define('DBPORT', 3306);
define('DBPREFIX', '');

$db = new mysqli(DBHOST, DBUSER, DBPASS, DBNAME);

if ($db->connect_error) {
    die('Connect Error (' . $db->connect_errno . ') '. $db->connect_error);
}

echo 'Converting the content from latin1 to utf8 ...' . PHP_EOL;

$table = 'ads';
$pk = 'id';
$latin1Column = 'content';
$failedColumn = 'failed';
//Creating a new failed column for store state
$db->query("ALTER TABLE {$table} ADD COLUMN {$failedColumn} TINYINT(1) UNSIGNED DEFAULT 0 NOT NULL AFTER {$pk}");

$limit = 1000;
$counter = 1;
$i = 0;
while($counter) {
    $counter = 0;
    $offset = $i * $limit;
    $sql = "SELECT SQL_NO_CACHE {$pk}, CONVERT(BINARY CONVERT({$latin1Column} USING latin1) USING utf8) AS content FROM {$table} ORDER BY {$pk} LIMIT {$offset}, {$limit}";
    if ($result = $db->query($sql)) {
        if ($result->num_rows) {
            $counter = $result->num_rows;
            while($rows = $result->fetch_assoc()){
                $id = $rows[$pk];
                $res = $db->query("UPDATE {$table} SET $latin1Column = '{$rows['content']}' WHERE {$pk} = {$id}");
                if(!$res) {
                    $db->query("UPDATE {$table} SET $failedColumn = 1 WHERE {$pk} = {$id}");
                }
            }
            $result->free();
        }
        $result->free();
    }
    $i++;
}

$db->close();