Uploading a file with PHP https://php.earth/docs/security/uploading

form.php

<form method="post" enctype="multipart/form-data" action="upload.php">
    File: <input type="file" name="file">
    <input type="submit">
</form>

upload.php

<?php

// Adjust where you want to upload a file. Best is to use non public location:
$uploadDir = __DIR__.'../uploads';

// Be sure the upload directory is writable
if (!is_writable($uploadDir)) {
    throw new \Exception('Error: The upload directory is not writable');
}

// Check if we've uploaded a file
if (!empty($_FILES['file']) && $_FILES['file']['error'] == UPLOAD_ERR_OK) {
    // Be sure we're dealing with an upload
    if (is_uploaded_file($_FILES['file']['tmp_name']) === false) {
        throw new \Exception('Error on upload: Invalid file definition');
    }

    // Rename the uploaded file
    $uploadName = $_FILES['file']['name'];
    $ext = strtolower(substr($uploadName, strripos($uploadName, '.')+1));
    $filename = round(microtime(true)).mt_rand().'.'.$ext;

    move_uploaded_file($_FILES['file']['tmp_name'], $uploadDir.'/'.$filename);
}