I am confused about the relation between the internal energy and temperature of ideal gases and single-phase materials. The following two statements from two distinct sources are mainly the reasons:
In single-phase, materials such as solids, liquids, and gases, the internal energy of a system depends primarily on the temperature.
quoted from Introduction to Thermal and Fluids Engineering by Deborah A. Kaminski and Micheal K. Jensen
... in an isothermal process, the internal energy of an ideal gas is constant. ... Note that this is true only for ideal gases; the internal energy depends on pressure as well as on temperature for liquids, solids, and real gases.
quoted from Isothermal Process, Wikipedia
Does the second article not suggest that they are single-phase when referring to liquids, solids and real gases? And also, why does the internal energy of single-phase materials depend primarily on temperature?