Please write 2 PHP scripts: sender.php and receiver.php. I'll explain the specs of those scripts below.

sender.php has a button labeled as "Send".
If you click the button, sender.php will send the zip file named sky.zip to receiver.php.
The path of sky.zip is "(the directory where sender.php resides)/data/sky.zip".
The file size of sky.zip is about 100KB.

After receiver.php receives sky.zip, it will save the sky.zip at the path,
"(the directory where receiver.php resides)/data/sky.zip".
If sky.zip already exists at the path, simply overwrite it.

Next, receiver.php pauses 0.5 second and then compute the digest of sky.zip.
Use md5_file() to compute the digest. Return the digest to sender.zip.

Now, sender.zip also computes the digest of sky.zip from the local copy of sky.zip and compares it with
the digest returned from receiver.php. If they match, sender.php
displays "Upload Completed!". If they don't match, sender.php will display
"Upload failed!".

That's it. If you have any questions about the spec, feel free to ask me.