I love pho and will soon try making it myself using an authentic Vietnamese recipe. I'm clear on all of the other ingredients and techniques, but "yellow rock sugar" has me stumped. Is this what the recipe means? If so, does it really taste different from regular sugar?
What's in the picture looks about right. This brand is commonly found in Asian grocery stores around here: (your mileage may vary). It is essentially crystallized cane sugar. If you really can't find it, you could probably substitute regular sugar. The flavour is a little different though, as well it isn't a refined white sugar.
According to "this guy" (random search), there is a difference and the yellow rock sugar often includes a few other sugars (unrefined brown sugar), and that you shouldn't ever use the clear/white rock sugar for pho.