Do ranks gained in a class count for purposes of enabling class skill bonus in another class?

Question

In Pathfinder class skills get a +3 bonus, as long as the character has one rank in a skill. Can this effect be activated by taking a rank in a skill while leveling up another class?

For example, a character takes her first level in Fighter, which has Survival as a class skill, but puts no ranks in it. At second level she takes a level in Cleric, and puts a rank in Survival, which is not a class skill for Cleric. Does she gain the +3 class skill bonus for fighter, even though the rank was taken while leveling Cleric?

Answer 1

Yes, she would

This isn't explicitly laid out in the rules, but it can be inferred.

In addition, each class has a number of favored skills, called class skills. It is easier for your character to become more proficient in these skills, as they represent part of his professional training and constant practice. You gain a +3 bonus on all class skills that you put ranks into.

Pathfinder doesn't force you to keep track of which ranks you got from which class, just the number you're getting per level. So, yes, that means that if you multiclass, all skills from both classes are considered class skills regardless of which class you're leveling at the time.

Consider the class skills part of your character's portfolio. Since your example Fighter above decided she'd learn the ways of being a Cleric, doesn't mean that during the process she's stopped being a Fighter, and as such could have been working on improving her survival (something her Fighter training has made her predisposed to) during the time learning how to be a Cleric.

Reference: http://www.d20pfsrd.com/skills#TOC-Acquiring-Skills