ZipArchive::addFromString() expects parameter 2 to be string, resource

I am making a page that will serve to decrypt .txt files that I will get from another website. I have been able to decrypt but only 1 to 1 so I am trying to decrypt 1 to 1 and put them all in a winrar. Unfortunately I'm having an error (post title) related to the "addFromString" method.

<?php

$zip = new ZipArchive();
$res = $zip->open('test.zip', ZipArchive::CREATE);
$directory = 'uploadFiles/';

    foreach (glob("uploadFiles/*.txt") as $file) {
    $decrypted = decrypt_file($file,'pass');
    $zip->addFromString($file, $decrypted);

    }

    if($res !== TRUE){
        echo $res;
    }

    $zip->close();



function decrypt_file($file,$passphrase){
    $iv = substr(md5("\x18\x3C\x58".$passphrase,true),0,8);
    $key = substr(md5("\x2D\xFC\xD8".$passphrase,true).md5("\x2D\xFC\xD8".$passphrase,true),0,24);
    $opts = array('iv'=>$iv, 'key'=>$key);
    $fp = fopen($file,'rb');
    stream_filter_append($fp, 'mdecrypt.tripledes', STREAM_FILTER_READ, $opts);
    return $fp;

  }

?>