Calling function in php page via GET?

I have a file that draws charts, called chart.php. Right now, I can call it using GET methods inside <IMG> tags.... <IMG src="chart.php?foo=bar"> Well, I'd like to be able to wrap it in a function, and call it via a function.... drawgraph(); and by GET syntax.... <IMG src="chart.php.drawgraph?foo==bar"> ?????? Is there a way to call a FUNCTION/METHOD in a php file using URLS? -Daniel