php centroid polygon

<?php
//-- array polygon
$polygon = array(array(50,50), array(75,25), array(105,55), array(125,75), array(100,100), array(75,125), array(50,100), array(25,75));
$er = polyCenter($polygon);
print_r($er);

function polyCenter($polygon){
  $x=$y=0;
  $n = count($polygon);
  for($i=0;$i<$n;$i++){
    $x+=$polygon[$i][0];
    $y+=$polygon[$i][1];
  }
  $x = round($x/$n);
  $y = round($y/$n);
  return $x.":".$y;
}
?>