test_imagecolorallocatealpha

<?php

$size = 300;

$image=imagecreatetruecolor($size, $size);

// 用白色背景加黑色边框画个方框
$back = imagecolorallocate($image, 255, 255, 255);

$border = imagecolorallocate($image, 0, 0, 0);

imagefilledrectangle($image, 0, 0, $size - 1, $size - 1, $back);

imagerectangle($image, 0, 0, $size - 1, $size - 1, $border);

$yellow_x = 100;

$yellow_y = 75;

$red_x    = 120;

$red_y    = 165;

$blue_x   = 187;

$blue_y   = 125;

$radius   = 150;

// 用 alpha 值分配一些颜色
$yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);

$red    = imagecolorallocatealpha($image, 255, 0, 0, 75);

$blue   = imagecolorallocatealpha($image, 0, 0, 255, 75);

// 画三个交迭的圆
imagefilledellipse($image, $yellow_x, $yellow_y, $radius, $radius, $yellow);

imagefilledellipse($image, $red_x, $red_y, $radius, $radius, $red);

imagefilledellipse($image, $blue_x, $blue_y, $radius, $radius, $blue);

// 不要忘记输出正确的 header！
header('Content-type: image/png');

// 最后输出结果
imagepng($image);

imagedestroy($image);

?>