Update multiple IDs at the same time

If you will update the field with the same value for all the id's, just use the example of Jéferson Bueno (in the comments):

update tabela set campo = 'valor' where id in ('id1', 'id2', 'id3')

If each id is given a different value, you will need to use multi-query.

Example :

<?php
$mysqli = new mysqli("localhost", "my_user", "my_password", "world");

/* verifica conexão */
if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
}

$query  = "update tabela set campo = 'Novo_a2' where id='id1';";
$query .= "update tabela set campo = 'Novo_c2' where id='id3';";
$query .= "update tabela set campo = 'Novo_d2' where id='id4';";

/* executa a sua multi query */
if ($mysqli->multi_query($query)) {
    do {
        /* Faz print separado do resultado de cada query */
        if ($result = $mysqli->store_result()) {
            while ($row = $result->fetch_row()) {
                printf("%s\n", $row[0]);
            }
            $result->free();
        }
        /* separa cada resultado por traços */
        if ($mysqli->more_results()) {
            printf("-----------------\n");
        }
    } while ($mysqli->next_result());
}

/* fecha conexão */
$mysqli->close();
?>