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You create a seven-layered sphere to protect an area. This multicolored sphere functions like a prismatic wall but is shaped in a 10-foot burst centered on a corner of your space. You must form the sphere in an unbroken open space so its edges don't pass through any creatures or objects, or the spell is lost.
Can you cast Prismatic Sphere when you are adjacent to a wall or on the ground? It seems like this spell would be difficult to use in most circumstances if you could not.
There is no formal definition of object in pathfinder, but Objects in combat are explained as a subheading of Movement in Encounters:
Because objects aren’t as mobile as creatures are, they’re more likely to fill a space. This means you can’t always move through their spaces like you might move through a space occupied by a creature. You might be able to occupy the same square as a statue of your size, but not a wide column. The GM determines whether you can move into an object’s square normally, whether special rules apply, or if you are unable to move into the square at all.
From this description it is clear that objects in combat are something "in" a space, like a statue or such, where you potentially still can move into the space, if they are not to big. They are not a solid wall that completely fills or acts as a boundary to available space.
Since a wall is not an object in that sense, the spell won't fail when you try to form it next to a wall, because
its edges don't pass through any creatures or objects
There is no creature or "object" there, just a wall.
Note that walls can be objects, even under the reading above. A wide column would count as an object. So corner cases, like a thin wall that you can get behind might be an object, and cause it to fail, too.
Is there support that walls in general should always be treated as objects? I think it is not clear cut. There are several lines of evidence for this, for example the material statistics rules talk about the hardness of objects in the context of a wall.
The material statistics rules are however primarily concerened with material-ness, not with object-ness, and do not use the name "Object" for the column that lists walls as example, they use "Item". They also refer to walls as "Structures". The sidebar on page 515 of the Core Rules talks about breaking doors, portcullises and walls and says
a portcullis made of iron, for example, has a higher Hardness than one of wood. For more on damaging objects, see page 272. Strong walls, such as well-maintained masonry or hewn stone, can’t be broken without dedicated work and proper tools.
You could read that as protcullises are objects, and can be broken using the object rules, and so are weak walls, while strong walls are not subject to those rules and cannot be broken, hence are not objects in the mechanical sense, or are unbreakable objects.
My take is that these rules are for playablity: instead of cleanly defining what an object is, and dealing with every bit of dungeon wall as an object, they are about how this plays out: if the walls are just the boundary of your battlemap, you ignore them as objects. If there is a low wall behind which an opponent is ducking that you want to blast away, or your players want to hack a hole through a wall, you think of it as an object.
If you ran with the evidence that every section of wall would always count as an object, then this spell would be difficult to cast. This would also include the floor, which is made of stone or wood too, which is even worse, as the spells area is a Burst which describes a globe, that would include the floor or ceiling, too:
issues forth in all directions from a single corner of a square within the range of the effect, spreading in all directions to a specified radius.
However, the templates next to the Areas section are just 2D. They do not care about above or below, because for the most part, that is not important for play and thinking about it just slows you down. So, for simplicity's sake, you ignore that the floor could count as an object and disrupt your spells, because you ignore vertical extension.
The same approach for simple, playable guides you to ignore ambient walls in combat for this spell, because I think the spell would be quite uselss if you could not cast it in a dungeon, and that can not be the intention of how a supposedly awesome high-level spell should work out. You want to protect yourself by casting this across a corridor, or by encircling a foe? Great, go right ahead. The exepmtion for creatures and objects is there so you cannot use the spell as an offensive weapon, and position the wall so you kill some enemies that get caught in it, or destroy their thrown weapons and ammunition.
It may be a bit unsatisfactory that some architectural features would cause the spell to fail, depending on a DM call. Or that the same feature could be treated as an object in some situations, and as not an object in others. However, I think the game has this ambiguity, for playability's sake.
Three ways to avoid it are to (1) use the hardline reading that every bit of wall, floor or ceiling is an object, and thus the spell is pretty much useless and you do never pick it, (2) you agree with the DM on a clear cut definition, like "walls to space that is physically inaccessible are not an object, other structures always are", or (3) you accept the read that normal walls are OK, and avoid casting the spell close to anything that is not plainly a wall.
If an object consists of more than one material, the GM typically uses the statistics for the strongest material involved. For instance, breaking a wall made of paper panels over a woven wooden framework would require damaging thin wood, not paper.
Walls and more problematically floors are objects, in that they have material statistics and object immunities. This means that the spell can only be effectively cast while in midair and away from any walls.
Allowing the spell to at least ignore floors is probably reasonable, given the severe limitation in use otherwise and that this was the explicit behavior in the previous edition:
Typically, only the upper hemisphere of the globe exists, since you are at the center of the sphere, so the lower half is usually occluded by the floor surface you are standing on.
