I was wondering if anyone could help me out in figuring the ratio part of the script. I don't know where to start.

I was thinking of just having an element in a mysql table that would count the number of impressions lets say - up to 100. Then there would be a table to count how many credits have been earned. And then with php when the count reaches 100, the code would add like 50 credits and then restart the count. Is that feasible or is there something more efficient?

Define "credits" as related to your intention. I will follow up on your other thread.

Credit would be the number of times their banner is displayed at other websites in the banner exchange. Specifically, it would be called 'unused credits' because it's the amount of times their banner will be displayed until they run out and end up at zero (until they get more impressions).

That's where implementing a rate of x impressions equals x amount of credits applied to their account gets tricky for me. I'm not sure how to go about doing it.

Hmmm...I'm not totally sure what you're asking, but let me take a wild guess and see if this at least helps.

Let's say that they get 50 credits per 100 impressions. Then, let's assume that the number of impressions is stored in a variable $impression_count (possibly pulled from a database field). Then, the total number of credits available would be floor($impression_count/100)*50. You could store the total number of credits used in another variable/field and that number would need to be always less than the bolded equation above.

For example, if the user had 248 impressions (i.e. $impression_count=248), then we'd have floor(248/100)*50 = floor(2.48)*50 = 2*50 = 100. If they had used 25 credits, then they'd have 100-25=75 credits available.

Hope that helps.

Thanks, got a good feeling about this one.