In a world where magic is not fully understood, being a spellcaster can be very difficult. For whatever reason, the people of this world do not yet have a full grasp of the power of magic. And with such lack of knowledge comes variation in the typical rule set.
0-th level spells can always be automatically learned, understood and recreated by anyone of the appropriate class, as normal. No special rules exist there.
Understand Spell
Spellcasters in a low magic world become adept at learning spells they see, feel or generally experience. With this in mind, the Spellcraft skill gains a new ability: Understand Spell. The spellcaster can attempt to understand the basics of a spell he experiences so as to learn it fully later. The difficulty of learning a spell as it is being cast is based on the level of the spell and the type of components it has. The difficulty of learning a spell which is already in place is only based on the level of the spell. Once a spellcaster understands a spell, he knows its level, components (but only if the spellcaster watched the spell being cast) and school but not any subschool or subtypes, unless he succeeds by 5 or more, in which case he knows all descriptors and subschools of the spell.
If the one casting the new spell does so without all components (through metamagic feats, for example) the spellcaster attempting to understand the spell does not learn the components, even if he succeeds by 5 or more on the Spellcraft check.
Recreate Spell
If a spellcaster succeeds on the Spellcraft check to understand the spell, he can attempt to recreate the spell later. He requires 2 hours per level of the understood spell to attempt to recreate the spell. During this time he experiments with various methods and formulas or prayers, as necessary, to recreate the spell. He needs to be in a quiet laboratory setting during this time but can take breaks as needed.
At the end of this time, he makes a Knowledge (arcana) check to Recreate Spell. The spellcaster can not attempt to recreate a spell he does not understand. The DC of the skill check is based on what the spellcaster already knows of the spell as well as the level of the spell. The base DC is 15 + level of the spell. If the spellcaster knows the components (as in he saw the spell being cast) the DC is reduced by 5. If the spellcaster knows all descriptors and subschools of the spell, the DC is reduced by 5. If the spellcaster knows the components and all descriptors and subschools, the DC is reduced by 10.
Rewards
Typically, from a meta gaming standpoint, to balance the playing field for a spellcaster in a low magic world, there should be some small reward for the PC for each spell which is recreated. If playing with the Disenchanting/Enchanting variant, perhaps there is a small amount of residuum as a byproduct of the spell recreation process which the spellcaster can collect. 100 ounces for each level of spell is a fair amount.
Each time a wizard gains a level, he automatically learns a small selection of spells. In the low magic variant, he learns an additional spell from his study in his free time. This reflects the amount of careful study spellcasters become adept at in this variant.