Empty byte array in PHP

I have a webapi that is written in c# that I need to send a request to from a PHP website. As part of the authentication scheme built into this webapi, I need to send an MD5 hash of an empty byte array. How would I do that in PHP? In C#, the following code works. I just need to do the same in PHP.

byte[] Content = new byte[0];
using (var md5 = MD5.Create())
{
  return md5.ComputeHash(content);
}