php - Temporary storage on a server -

i'm looking efficient , secure way store small amount of user data line of text, indefinite period of time. scenario: 1 client 'a' sends data client 'b' , asynchronously via server 's'. simplicity, consider data single line of text. now, data delivered client 'b' server 's' , only when b asks it. until time server 's' has store data.

for simple demo, implemented crudely follows:
client 'a' post request xmlhttprequest, sending text 's' follows

    xhr.open("post",url+"?senttext=text"); //url points php file on s     xhr.send();  

the php code saves received text file follows:

    <?php     $selected = $_get["senttext"];     $writefile = "text.txt";     $fh = fopen($writefile,'w');     fwrite($fh,$selected);     fclose($fh);     ?> 

later, after amount of time, b asks text in file,using bufferedinputstream follows:

    url fileurl = new url("url/text.txt");     bufferedreader in = new bufferedreader(new inputstreamreader(fileurl.openstream()));     while((tmpreceived = in.readline())!=null){        received = tmpreceived;     } 

this demo works properly, demo. however, if full scale real world application millions of users, how implement following:

the part server s stores text. obviously, can't save text plain text in file, breach users' privacy. , ineffecient when there millions of users. how commercial services dropbox achieve this?

to keep things simple, need know how text, can extrapolate other types possibly.

thanks lot!