dropzone.php

<?php
// Source: https://www.php.net/manual/en/features.file-upload.php
header('Content-Type: text/plain; charset=utf-8');
try {
    if (
        !isset($_FILES['file']['error']) ||
        is_array($_FILES['file']['error'])
    ) {
        throw new RuntimeException('Invalid parameters.');
    }

    switch ($_FILES['file']['error']) {
        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.');
    }

    if ($_FILES['file']['size'] > 1000000) {
        throw new RuntimeException('Exceeded filesize limit.');
    }

    $finfo = new finfo(FILEINFO_MIME_TYPE);
    if (false === $ext = array_search(
            $finfo->file($_FILES['file']['tmp_name']),
            array(
                'jpg' => 'image/jpeg',
                'png' => 'image/png',
                'gif' => 'image/gif',
            ),
            true
        )) {
        throw new RuntimeException('Invalid file format.');
    }
    if (!move_uploaded_file(
        $_FILES['file']['tmp_name'],
        sprintf('data/'.uniqid().'.png',
            sha1_file($_FILES['file']['tmp_name']),
            $ext
        )
    )) {
        throw new RuntimeException('Failed to move uploaded file.');
    }
    echo 'File is uploaded successfully.';
} catch (RuntimeException $e) {
    echo $e->getMessage();
}