PHP — Convert milliseconds to Hours : Minutes : Seconds.fractional

I've got a script that takes in a value in seconds (to 2 decimal points of fractional seconds):

$seconds_input = 23.75

I then convert it to milliseconds:

$milliseconds = $seconds_input * 1000; // --> 23750

And then I want to format it like so:

H:M:S.x // --> 0:0:23.75

Where 'x' is the fraction of the second (however many places after the decimal there are).

Any help? I can't seem to wrap my mind around this. I tried using gmdate() but it kept lopping off the fractional seconds.

Thanks.