Using PHP make a JSON file that is Dynamicaly shown with AJAX

I have a database already given to me, Using PHP I need create a JSON File that gets Data From the database that only works on a get request ?catname="example" Then in a separate page i need to use jquery and ajax to create a drop down menu the gets the category names from the database. Then when the category name is selected use an on change event to show the data in table form in that category only So in short I need to return the value on the select box use .getJSON on the url I defined at [login to view URL] = ? pass the value of the select box to the end of the url I are requesting with the [login to view URL] so, .getJSON ([login to view URL] = selectboxValue) then pull back a json array, then I need to do an each in jquery around the values of the array to display them in a table (or something) use append to add the values to the end of the container ill want them stored in I can supply you with all the code I have so far including the MYSQL Database and the php code I am using to create the JSON File Thank You