You need to open your FLA in Flash, and add a dynamic text box where you want it to show. You then have to name the textbox in the properties dialog box. Then you need to ad some AS to your file. In an AS layer, you would use something like:
root.myVar = root.urlVariable;
myVar being the name of the textbox and urlVariable being the name of the variable value that you will pass from the query string. Then in your SWFObject <SCRIPT> tags, add the so.addVariables using the urlVariable name - which will grab the value and put it into the SWF at runtime, so now your empty myVar has the value from the query string.
Again, if you are not familiar with Flash, you need to go to the Flash forums for details on how to understand those instructions. Your question really doesn't have anything to do with PHP, but has to do with using the Flash authoring tool.
Hope that helps...