A utility function to calculate area of triangle formed by ( x1 , y1 ) , ( x2 , y2 ) and ( x3 , y3 )
< ? php function area ( $ x1 , $ y1 , $ x2 , $ y2 , $ x3 , $ y3 ) { return abs ( ( $ x1 * ( $ y2 - $ y3 ) + $ x2 * ( $ y3 - $ y1 ) + $ x3 * ( $ y1 - $ y2 ) ) / 2.0 ) ; }