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you can't use same variable for both category and sub-category as i think, both will be having different values..in this case, you just need to use following code:
$category=$_POST['Category'];...

to use the function as suggested by George, you need to use that button also and use onclick event handler to call the ajax javascript function

you can also do it by javascript..by using onclick event handler and a common function for both buttons to handle the event

you can get the information entered on first page through $_GET("fieldname") on second page and display it in a form with those fields disabled with back and submit button.When user click submit...