Casters are the classes that need to know the rules of their spells, but even for them it is not required upfront knowledge. It is possible to "learn by doing" in a "tutorial mode" way. Reading the PHB is needed for them to get an idea of the kind of character they would like to have. A rude and quick Barbarian, or a master of the fine arts of magic ?
When I start a new campaign, I organize a meeting with each of them, just the two of us. They can go through the rulebook and read it "by pictures" and have their imagination run wild. Once they get an idea of the role they want to interpret, we roll the dice and fill up the character sheet together. This gives them a brief introduction to the rules. Occasionally, you can organize a quick "phony session" lasting 10 minutes or even 30 seconds to explain a particular mechanics (such as initiative or ability checks).
The first all-together session will be light and with maybe just one small fight, so that they can get into the mechanism slowly. In two or three sessions, they will learn the language and know what to do if you ask them "to do a check for equilibrium". Eventually they will be confused with the die to use, but it's a quick fix. The mechanics and rules will be learned by repetition and direct application.
From there, they can delve into the PHB if they want, but it's not mandatory. You will introduce them to the need to learn a bit more when they level up. It's likely they will level up all at the same time, so half of a session will go for level up. Ideally, if you have an experienced player who can help others it will be faster, otherwise delay the level up to the next session, and tell them different times (by ten minutes) they can be there, so that you can level them up progressively as they arrive. While you level up the newly arrived, those who are already done can put their nose in the PHB or chat about new things their character can do, so they won't get bored.