s3 direct upload restricting file size and type

To do this, I somehow need to list all files in an amazon bucket's folder, and find the one which has been added last.

S3's API isn't really optimized for sort-by-modified-date, so you'd need to call list_buckets() and check each timestamp, always keeping track of the newest one until you get to the end of the list.

An automatic PHP script detects that a new file has been added and notifies clients

You'd need to write a long-running PHP CLI script that starts with:

while (true) { /*...*/ }

Maybe throw an occasional sleep(1) in there so that your CPU doesn't spike so badly, but you essentially need to sleep-and-poll, looping over all of the timestamps each time.

I've tried $s3->list_objects("mybucket");, but it returns the list of all objects inside the bucket, and I don't see an option to list only files inside the specified folder.

You'll want to set the prefix parameter in your list_objects() call.

You are passing an array to exif_imagetype(), it should be a string with filename, for your example, you should iterate through your array or use something like:

// Get type from first image
$detectedTypeImage1 = exif_imagetype($tmp_name_array[0]);
$detectedTypeImage2 = exif_imagetype($tmp_name_array[1]);

Then you are wrongly compare an array with an int

$size_array > 2097152

you could do like the first example I gave or you could use a loop through the array, like:

foreach($size_array as $imageSize){
    if($imageSize > 2097152){
        echo "type error SIZE";
        echo "<br>";
    }
}

Full example

/**
    * Reorder the uploaded files array to simply the use of foreach
    * 
    * @param array $file_post
    * @return array
    */
    function reArrayFiles(&$file_post) {

    $file_ary = array();
    $file_count = count($file_post['name']);
    $file_keys = array_keys($file_post);

    for ($i=0; $i<$file_count; $i++) {
        foreach ($file_keys as $key) {
            $file_ary[$i][$key] = $file_post[$key][$i];
        }
    }

    return $file_ary;
}



// Array reArranged
$file_ary = reArrayFiles($_FILES['file_array']);

$allowedTypes = array(IMAGETYPE_PNG, IMAGETYPE_JPEG, IMAGETYPE_GIF);


// For each file...
foreach($file_ary as $key => $file){

    static $i = 1;

    if($file['error'] == 4){
        echo 'No file uploaded on field '.$i++.'.<br>';
        continue;
    }

    $errors = false;

    // Check type, if not allowed tell's you wich one have the problem
    if(!in_array(exif_imagetype($file['tmp_name']), $allowedTypes)){
        echo '<span style="color:red">Filename '.$file['name'].' <b>type</b> is not allowed.</span><br>';
        $errors = true;
    }

    // Check size, if exceed allowed size tell's you wich one have the problem
    if($file['size'] > 2097152){
        echo '<span style="color:red">Filename '.$file['name'].' exceeds maximum allowed <b>size</b></span>.<br>';
        $errors = true;
    }


    // If we don't have errors let's upload files to the server
    if(!$errors){
        if (move_uploaded_file(
                $file['tmp_name'], "../upload/" . $file['name'])
        ){
            echo $file['name'] . " upload is complete<br>";

        }else{
            echo "Uploaded failed for " . $file['name'] . "<br>";
        }   
    }
    $i++;
}

Don't forget to use is_uploaded_file() too.