How to transform this code so it validates multiple files?

First you have to make sure that the FILE fields of the form are "Array":

<form action="file-upload.php" method="post" enctype="multipart/form-data">
  Envie esses arquivos:<br />
  <input name="upfile[]" type="file" /><br />
  <input name="upfile[]" type="file" /><br />
  <input type="submit" value="Enviar arquivos" />
</form>

When the above form is submitted, the $_FILES['upfile'] , $_FILES['upfile']['name'] , and $_FILES['upfile']['size'] array will be placed in an array $_FILES :

Array
(
    [name] => Array
        (
            [0] => foo.txt
            [1] => bar.txt
        )

    [type] => Array
        (
            [0] => text/plain
            [1] => text/plain
        )

    [tmp_name] => Array
        (
            [0] => /tmp/phpYzdqkD
            [1] => /tmp/phpeEwEWG
        )

    [error] => Array
        (
            [0] => 0
            [1] => 0
        )

    [size] => Array
        (
            [0] => 123
            [1] => 456
        )
)

In the code you will have to put a for to go through it and make the uploads:

<?php

header('Content-Type: text/plain; charset=utf-8');

for($i = 0; $i < count($_FILES['upfile']['name']); $i++){

try {

    // Undefined | Multiple Files | $_FILES Corruption Attack
    // If this request falls under any of them, treat it invalid.
    if (
        !isset($_FILES['upfile']['error'][$i]) ||
        is_array($_FILES['upfile']['error'][$i])
    ) {
        throw new RuntimeException('Invalid parameters.');
    }

    // Check $_FILES['upfile']['error'] value.
    switch ($_FILES['upfile']['error'][$i]) {
        case UPLOAD_ERR_OK:
            break;
        case UPLOAD_ERR_NO_FILE:
            throw new RuntimeException('No file sent.');
        case UPLOAD_ERR_INI_SIZE:
        case UPLOAD_ERR_FORM_SIZE:
            throw new RuntimeException('Exceeded filesize limit.');
        default:
            throw new RuntimeException('Unknown errors.');
    }

    // You should also check filesize here. 
    if ($_FILES['upfile']['size'][$i] > 1000000) {
        throw new RuntimeException('Exceeded filesize limit.');
    }

    // DO NOT TRUST $_FILES['upfile']['mime'][$i] VALUE !!
    // Check MIME Type by yourself.
    $finfo = new finfo(FILEINFO_MIME_TYPE);
    if (false === $ext = array_search(
        $finfo->file($_FILES['upfile']['tmp_name'][$i]),
        array(
            'jpg' => 'image/jpeg',
            'png' => 'image/png',
            'gif' => 'image/gif',
        ),
        true
    )) {
        throw new RuntimeException('Invalid file format.');
    }

    // You should name it uniquely.
    // DO NOT USE $_FILES['upfile']['name'] WITHOUT ANY VALIDATION !!
    // On this example, obtain safe unique name from its binary data.
    if (!move_uploaded_file(
        $_FILES['upfile']['tmp_name'][$i],
        sprintf('./uploads/%s.%s',
            sha1_file($_FILES['upfile']['tmp_name'][$i]),
            $ext
        )
    )) {
        throw new RuntimeException('Failed to move uploaded file.');
    }

    echo 'File is uploaded successfully.';

} catch (RuntimeException $e) {

    echo $e->getMessage();

}

}

?>