If subjected to multiple concentration checks, are all rolled separately?

Question

What happens if multiple concentration checks are required from different sources while a caster is trying to cast a spell? For example, if we have a (very unfortunate) wizard who is trying to cast Freedom of Movement while concurrently:

1. On fire, taking 1d6 every round (on average a DC 16 concentration check)
2. Grappled by a Giant Squid (CMD of +22, so a DC 36 concentration check)
3. Entangled ( a DC 19 concentration check)
4. Casting Defensively ( a DC 23 concentration check)

...What rolls does the hapless wizard have to attempt?

I've seen various posts on the paizo forums and reddit stating all manner of conflicting answers, ranging from stacking DCs together, only caring about the highest, and (what seems closest to RAW) rolling against each effect. What is the correct number and DC of concentration rolls in this scenario? And, failing official rules, I'm sure that the PFS has a mandate on how to handle such a situation, though I cannot find one.

Answer 1

From the srd:

To cast a spell, you must concentrate. If something interrupts your concentration while you're casting, you must make a concentration check or lose the spell.

From this, stacking DCs is clearly wrong but either of the others is a perfectly valid interpretation. Therefore it is a DMs call: does Concentration block out all distractions or does each distraction require an effort by the spellcaster to ignore?

To inform your decision making, please feel free to use this anydice script. Just change the MOD to reflect the spellcaster's Concentration check bonus and change the list of DC you want to check.