PHP fmod returns the dividend not remainder

Question

This returns 50727.64 instead of the expected 0.489

fmod((float)50727.64, (float)103680);

Is this because the normal division only returns a decimal?

This code is for a formula to calculate full containers and any partials but appears to not work if the container would be only 1 partial container. In this example, the 103680 is the container size and 50727.64 is the contents. The contents may be any amount above 0 to fill in any number of containers.

Is there another way to extract just the remainder.

I know I can check if the dividend is smaller than the divisor and just use that result of regular division but it seems pointless to add the extra code and check.