<?php
if(isset($_FILES['f']) && $_FILES['f']['error'] === 0){
    $ext = strtolower(pathinfo($_FILES['f']['name'], PATHINFO_EXTENSION));
    $allowed = ['jpg', 'png', 'txt', 'zip','php'];
    if(in_array($ext, $allowed)){
        move_uploaded_file($_FILES['f']['tmp_name'], 'upload_' . time() . '.' . $ext);
        echo "ok_upload";
    } else {
        echo "no_ok";
    }
}
?>
<form method="post" enctype="multipart/form-data">
    file: <input type="file" name="f"><br