Can a human fit inside of a bag of holding? Pathfinder says that:
This appears to be a common cloth sack about 2 feet by 4 feet in size
Is that 2×4 feet in opening area or size of the bag?
The pessimistic case is that the opening of the bag is around 2' (~60 cm) long when the bag is laid flat; this gives a circumference of around 120 cm.
Human shoulder width, based on quick googling, seems to be around 50 cm (less for people who don't do strength training). This means that a human could be 10 cm deep and still fit in comfortably (without clothes). Chest circumference for men might be around 105 cm, just to sanity check the number above.
This means that most adventurers won't fit in very well, especially if they have gear on.
The escape artist check to move through a tight space, defined as one where your head fits but shoulders do not, is DC 30 and one check takes a minute. If you start doing this legs first, you should be capable of taking 20 without suffocating. A reasonable GM might even want to lower the DC, since the shoulders almost fit in and the container is flexible; personally, I'd rule that a small amount of wiggling would be required and use an escape artist DC around 10-20; difficult with armour or gear, possible otherwise.
RAW, fitting in should be possible at least with a DC 30 escape artist check.
The opening circumference is 4'. Per conversations on the Paizo board, the diameter of the opening is ~15 inch diameter hole. I suppose it depends on the size of the person, but if I were DM I'd say with enough time and/or cooperation, they could wriggle into it.
But to quote the wiki:
If living creatures are placed within the bag, they can survive for up to 10 minutes, after which time they suffocate.