You could have one page, let's call it auto_bid.php. This file could be a basic php file that receives a string through GET or POST, does a curl to wherever and receives the JSON response. You could process the response and generate your own response.
Your bid.php file could have the input box and button. When the user enters criteria and clicks the button, it would make a call to a javascript function that is set on a recursive 3 second timeout loop (settimeout() perhaps), and that function can use a javascript or jquery ajax call (jquery has a built in method) passing the string, to auto_bid.php.
The response from auto_bid.php that you set up could be a JSON string that you can have js handle and adjust the bid.php html to display "auto bidded $XX". Then it would attempt a bid again and display again. You could also have auto_bid.php respond with HTML instead, and when you receive it through your ajax call, just dump it into a DIV or something.
First of all, could you provide me with some clarification: are you coding a chat room (one-to-many) or individual (one-to-one) chats?
There are a couple of obvious flaws in your current system's design that I would like to point out. First, I'll start out with a brief analysis and then explain what is wrong with it and what you can do to fix it. Obviously if you want to run a successful project you should start from step one and get your hands dirty with systems analysis and design.
Problem
Website receives a high volume of traffic and eventually crashes.
Requirements
- Sustain high volume of web traffic
- Display previous 150 messages
- Secure communication pathway between clients
Problem Analysis
Right away it is obvious that your site is crashing because the code that opens the file is being called hundreds of thousands of times per second. Opening files and writing to them is very memory intensive.
The FILE Probem: Files do not handle concurrency very well at all. In fact, they're terrible with concurrency. Think about it like this, essentially you've opened the same file with notepad with hundreds of thousands of open windows/processes and you're changing the content in all of them simultaneously. When you try to solve a problem with this type of solution you end up with non-deterministic results. Basically, it is impossible to predict what data will be in the file.
Fortunately, there is a way to get deterministic results while still using files if you lock them properly. Unfortunately, this is not a solution to your problem. In your case, only one person would be able to send a message at a time. Surely that is NOT the solution you want!
Wait... there IS a solution:
You CAN USE a Database!
Databases are particularly good at solving this sort of concurrency issue! Depending on what database/engine you use your table may lock or only a single record might lock. In you case, I would suggest a free database like MySQL and a record-locking engine like InnoDB. If you're not a database rookie, you might want to look into MariaDB as well, it is a fork of the MySQL project by the original developer and is a binary drop in.
Basically, there is no way around it using a database for this type of solution. In fact, databases are very powerful and you can program procedures with them. From your query you can choose to select only 150 messages and then order them by most recent very easily. All users will be able to send messages at the same time with a record-locking database engine like InnoDB.
I would like to additionally point out that I would be a little troubled to find out what the code for the rest of the application looks like. It is very easy to write PHP code that looks fine but performs terribly. I'm not sure if you're familiar with asymptotic analysis or unit testing but I highly suggest that you thoroughly test your code before pushing it to production. Given the size of your userbase you should be concerned about code optimization and runtime. If your application/problem/solution was properly analyzed, designed, implemented, tested, and debugged you would have a much better handle on your problem.
It is also very easy to write code in PHP that is insecure. I would like to advise you to test your code thoroughly (try to break it) when writing modules that interact with the database. Poorly coded web applications can be very easy to exploit and given your user base of 300-400k I wouldn't doubt it if Cindy Lou Who suddenly decided to give it her own security audit. If a white hat hacker discovers the flaw in your system they will likely encourage you to fix it. If a black hat hacker discovers the flaw they will likely use it to spread malware and steal information.
Best Answer
The biggest thing that I see is missing is keeping track of what the user has actually paid. Some people pay an amount more or less than the amount due, so some may have a balance due from the last invoice while others may have pre-paid for several periods ahead.
EDIT: Based upon your comment, I see this is handled seperately, great!
The other thing I see is the way you are handling the TIMESTAMP field. For example:
Paid monthly
if ( Timestamp is in past = true AND Month gone by = 0 AND Days gone by >= 20 )
then ( create a new invoice and set "Last Due Date" to time() )
Suppose I initially signed up on 1/1/2012, then Timestamp starts off with that date. Assuming I pay monthly, this will mean you generate an invoice on 1/20/2012 and set the Timestamp to 1/20/2012. Does this mean you generate an invoice every 20 days rather than once per month? In other words will you generate an invoice on 2/9/2012 (20 days after 1/20/2012)?
The point is that the next invoice should be generated based upon the end date of the current billing period, not the date the invoice was generated.
Currently you "set "Last Due Date" to time()", perhaps you want to set Last Due Date to the first day of the next billing period? So for example when generating monthly invoices on 1/20/2012 you would change 1/1/2012 (current value in DB) to 2/1/2012. You don't state which database you are using, but many have built-in functions to add 1 month, quarter, year etc.