Drawing mandalas with PHP

mandala.php

<?php
// Prints a mondala
$points = $_GET['points'];
$size = $_GET['size'];
// Test the size
if($size < 1 || $size > 1024){
	echo "Please use a better size";
// Test the number of points
}elseif($points < 1 || $points > 100){
	echo "Please use a more reasonable number or points";
}else{
	// Set the radius to half of the size of the image, so it fills the image
	$radius = $size/2;
	// Find the number of degrees between points (in radians)
	$degrees = deg2rad(360/$points);
	// The offset used to push the whole drawing into the first quadrant (because our math would 
	//		normally put it centered on 0,0
	$offset = $size/2;
 
	// Create the image
	$im = imagecreatetruecolor($size, $size);
	// Set the background color white
	$background = imagecolorallocate($im, 255, 255, 255);
	imagefill($im, 0, 0, $background);
	// Set the color of the lines black
	$lines = imagecolorallocate($im, 0, 0, 0);
 
 
	// Until we have made our way around the circle
	while($angle < deg2rad(360)){
		// Increase the angle to point to the next point
		$angle = $angle + $degrees;
 
		// Find the new points
		$x = $radius * cos($angle) + $offset;
		$y = $radius * sin($angle) + $offset;
		// Add those points to the array of points
		$pointsArr[] = array($x, $y);
	}
 
	// For each point
	while($from = array_pop($pointsArr)){
		// For each point that has not yet been plotted
		foreach($pointsArr as $to){
			// Draw a line
			imageline($im, $from[0], $from[1], $to[0], $to[1], $lines);
		}
	}
 
	// Output the image
	header ("Content-type: image/png");
	imagepng($im);
	// Clear the image form memory
	imagedestroy($im);
}
?>

Fatal error: Uncaught Error: Call to undefined function imagecreatetruecolor() in D:\xampp\htdocs\MANDALA\mandala.php:21 Stack trace: #0 {main} thrown in D:\xampp\htdocs\MANDALA\mandala.php on line 21