Compute partial polygon area

[Krung Saengpole]

Hi all,
   
  I'd like to compute partial area of polygon that intersects with another polygon. Is there a way to do with PHPMapscript? For example, I want to know how many square kilometers of forests that reside in the area of 50km-radius circled area. Or where can I get to the example?
   
  Thank you in advance.
  Krung

		
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