Description:
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The gd extension function imagepolygon() requires at least 3 points.  While this restriction is documented for imagefilledpolygon(), it is not documented for imagepolygon(). Since both are implemented with the same function php_imagepolygon() and contain the same check, the documentation for imagepolygon() should be updated to match imagefilledpolygon() in requiring at least 3 points.

However, this is not just a documentation problem. The implementation does not actually check for num_points >= 3 as the documentation for imagefilledpolygon says. It checks that the points array size is at least 6 (3 points, 2 values each). So you can plot a 2-point or even 1-point filled or unfilled polygon, if you use an array with 3 or more points in it but specify num_points=2 or 1.  This is very strange, not working as documented, and surely not as intended.

But there's more. As far as I can tell, libgd (bundled or stand-alone) doesn't actually have a 3-point minimum restriction to gdImagePolygon() or gdImageFilledPolygon(), although it does seem to be documented. See the sample code below, which plots 2-point filled and unfilled polygons by using the above trick. It also works with num_points=1.  If true, the PHP check for 3 or more points is not needed at all. 

Test script:
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<?php
# Plot illegal 2-point polygon and filled polygon using over-sized array.
$g = imagecreate(300, 300);
$bgnd = imagecolorallocate($g, 255, 255, 255);
$fgnd = imagecolorallocate($g, 0, 0, 0);
imagefilledpolygon($g, array(100,10, 100,100, 180,100), 2, $fgnd);
imagepolygon($g, array(200,10, 200,100, 280,100), 2, $fgnd);
imagepng($g);


Expected result:
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An error, since num_points=2 is not valid per documentation of ImageFilledPolygon.

Actual result:
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A PNG image with 2 lines (2-point polygons).