What is yellow rock sugar, for pho?
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.