<?php

    if(isset($_FILES['avatar'])){

        // Let's store that bad boy in a variable
        $img = $_FILES['avatar'];

        // In what folder do we want to save the file in?
        $folder = "uploads";

        // Let's find that extension
        $extension = pathinfo($img['name'], PATHINFO_EXTENSION);

        // Generate a 'random' name
        $randomFileName = time() . ".$extension"; // Example: 123124124.png

        // The new generated file name that points to the location that we want the file to be in
        $fullFileName = $folder . "/" . $randomFileName; // Example: uploads/123124124.png

        // If it has to be an IMAGE:
        $isImage = getimagesize($img['tmp_name']);
        if(!$isImage) {
            $error = "The file has to be an image!";
        }

        // If it has to be an IMAGE:
        $size = filesize($img['tmp_name']);
        if($size > 2097152) { // 2MB
            $error = "This file is too large. It must be smaller than 2MB!";
        }

        // If there are no errors, let's move the file
        if(!isset($error)){

            // Try to move the file
            $upload = move_uploaded_file($img['tmp_name'], $fullFileName);
            if($upload){
                // Yay! It is uploaded.
            } else {
                $error = "File could not be uploaded. Such a shame!";
            }
        }
    }

?>

<form action="upload.php" enctype="multipart/form-data" method="POST">

    <?php if(isset($error)) echo $error ?>

    <input type="file" name="avatar">
    <input type="submit">
</form>