Keyword notify mod for php classifieds script

I need a keyword notification mod for a classifieds site. The script is [login to view URL] An email message is sent to someone when an ad or event has been placed with specific keywords in the title or description. The user will need to be able to create a free membership account first. Within their account they can enter keywords that they wish to monitor. Each keyword will have a cost to it. Easiest way to sell the keywords would be through credits. They could purchase a package of credits for X amount. Each keyword they enter a credit is deducted. Keywords could be good for X amount of days like 30, 60, 90 days then the credit expires. The credits can be used for ads and events. It can be switched to free or paid in the [login to view URL] file of the classifieds script. In the admin area I need to manage users, credit package pricing including number of credits for X amount of money. I also need to be able to manage user credits just in case I want to add some to their account for free. It should work with PayPal. Each sale will be reported in the admin area under the payment history section. Keywords that user could enter are like "ford mustang" "pine chair" surfboard etc. Max number of words in a string could be set by admin. For example "red ford mustang with custom wheels" would be 6 word so I may want to limit it to 3 only like "red ford mustang". A cron job needs to be created to run at a specific time of day to check for new ads with matching emails then send out email notifications to matching keyword accounts. The email would contain a message alerting them that an ad has been posted with their keyword and have a link for them to click on to see the ad. I will need step by step instructions on how to install this mod into the classifieds site. Once the user has signed up for a membership they can sign in a and place classifieds without having to click on an email verification link that they normally get when placing ads. The admin area needs to be flexible as I will have more mods coming soon, this same admin area will be where we will manage all the features. Right now the script has it's own admin area but I will need a link under the tools section which opens up a separate admin area.