Hey
Brack, welcome to the boards
To answer your question: yes and no
d20, and D&D specifically, has undergone a few changes in the transition from 3.0 (released in 2000) to 3.5 edition (released 2003). This means that "d20" materials could have been changed in this transition - take somehting like the Monks' unarmed damage progression, or the Ranger picking up a second option for combat style (archery, in this case).
When it comes to specifics, you will likely find a few differences depending on who published what when. Usually, the more recent version of the most applicable book wins.
Say something in the DMG contradicts the way a spell or feat works in the PHB - well, the PHB is the place where feats and spells are explained, and thus whatever is in the PHB wins. Now say that the DMG was 3.5, and the PHB was 3.0. Well, you'd have to get a copy of a 3.5 PHB in order to see if that feat/spell/etc has changed in the edition, and if so if there is still a contradiction. If it is still there, the PHB would win.
So, if you have any specific questiosn about stuff, fell free to ask around the boards here - we're always here to help! But for broad strokes, d20 is the same mechanic over and over, and so it shall remain
cheers,
--N