如何在页面重新加载时保留表单的内容？

I have a php form where user enters data and in between uploads multiple documents.

Every time user selects a file and clicks on upload prior to pressing the submit button is the following code:

if ( isset($_POST['Upload'] ) ) {
    //upload code is here
}

The problem is that the page reloads everytime the user clicks on upload and all the data entered in the textbox is wiped off.

It is required that the data already entered into the form be there after the file is uploaded and the page reloads.

Should I use:

isset($_POST['Upload'] )

or should I use a session variable to store the content of the textbox?