PHP验证并将信息存储到会话变量中

I'm setting up 2 text-boxes and one drop down options list in html. I'm trying to use php to validate if these text box are filled in or not. If clicked on the submit button ('btnCalculate'), the validation occurs, if nothing in the text boxes, there will be an error message that pops up beside it ('pamountErrorMsg, irateErrorMsg'). If there is substance that is typed into the textboxes, the browser will save the information into Session["pamount"] or "irate". I am having trouble validating the blank text boxes and adding the variables into Session before I can use them in the next page. Right now it is not validating anything at all. What is wrong with my syntax, if any?I'm setting up 2 text-boxes and one drop down o