Para quem gostaria de usar ou entender como usar este fantástico recurso do PHP, a biblioteca GD.

USO:
<img src="http://server/pie3D.php?total=45&usado=23">

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Esconder código-fonte

<?php
$sum = $_GET['total'];
$degrees = Array();
$diameter = 200;
$radius = $diameter/2;

$slice[1] = $_GET['total'] - $_GET['usado'];
$slice[2] = $_GET['usado'];

$neg = $slice[1]<0 ? -1: 1;

if ($neg == -1){
    $slice[1] = 0;
    $slice[2] = $_GET['total'];
}

//Converte para graus
for ($y=1; $y<=2; $y++)
{
$degrees[$y] = ($slice[$y]/$sum) * 360;
}

Header("Content-Type: image/png");

require ('config.inc.php');

$im = ImageCreate(450, 300);


//Pie Colors
//Fill
$cor_01['R'] = 255;
$cor_01['G'] = 255;
$cor_01['B'] = 255;
//Free color
$cor_02['R'] = 177;
$cor_02['G'] = 201;
$cor_02['B'] = 237;
//Used Color
$cor_03['R'] = 210;
$cor_03['G'] = 210;
$cor_03['B'] = 210;


//Cores
$white = ImageColorAllocate($im, 255, 255, 255);
$black = ImageColorAllocate($im, 0, 0, 0);
$cinza = ImageColorAllocate($im, 102, 102, 102);

$cor01 = ImageColorAllocate($im, $cor_01['R'],$cor_01['G'],$cor_01['B']);
$cor02 = ImageColorAllocate($im, $cor_02['R'],$cor_02['G'],$cor_02['B']);
$cor03 = ImageColorAllocate($im, $cor_03['R'],$cor_03['G'],$cor_03['B']);

if ($neg == -1){
    $cor03 = ImageColorAllocate($im, 255, 50, 50);
}
// preenche o circulo
ImageFill($im, 0, 0, $cor01);

// desenha a linha base
ImageArc($im, 153, 153, $diameter, $diameter,315,135,$black);
ImageLine($im, 150,150, 225, 150, $black);

for ($z=1; $z<=2; $z++)
{
// calula o arco
ImageArc($im, 150, 150, $diameter, $diameter, $last_angle,($last_angle+$degrees[$z]), $black);
$last_angle = $last_angle+$degrees[$z];
// calculate coordinate of end-point of each arc by obtaining
// length of segment and adding radius
// remember that cos() and sin() return value in radians
// and have to be converted back to degrees!
$end_x = round(150 + ($radius * cos($last_angle*pi()/180)));
$end_y = round(150 + ($radius * sin($last_angle*pi()/180)));
// demarcate slice with another line
ImageLine($im, 150, 150, $end_x, $end_y, $black);
}

// this section is meant to calculate the mid-point of each slice
// so that it can be filled with colour
// initialize some variables
$prev_angle = 0;
$pointer = 0;
$zz = 1;

for ($z=1; $z<=2; $z++)
{
// to calculate mid-point of a slice, the procedure is to use an angle bisector
// and then obtain the mid-point of that bisector
$pointer = $prev_angle + $degrees[$z];
$this_angle = ($prev_angle + $pointer) / 2;
$prev_angle = $pointer;
// get end-point of angle bisector
$end_x = round(150 + ($radius * cos($this_angle*pi()/180)));
$end_y = round(150 + ($radius * sin($this_angle*pi()/180)));
// given start point (150,150) and end-point above, mid-point can be
// calculated with standard mid-point formula
$mid_x = round((150+($end_x))/2);
$mid_y = round((150+($end_y))/2);
// depending on which slice, fill with appropriate colour
if ($z == 1)
{
ImageFillToBorder($im, $mid_x, $mid_y, $black, $cor02);
}
else if ($z == 2)
{
ImageFillToBorder($im, $mid_x, $mid_y, $black, $cor03);
}
}

//Legenda.
$ret_x01=300;
$ret_y01=40;

$slice[1] = round($slice[1]/$_GET['total'] * 100,1);
$slice[2] = round($slice[2]/$_GET['total'] * 100,1);

imagefilledrectangle($im, $ret_x01,$ret_y01,$ret_x01 + 15 ,$ret_y01 + 15,$cinza);
imagerectangle($im,$ret_x01 - 3 ,$ret_y01 - 3,$ret_x01 + 12 ,$ret_y01 +12,$black);
imagefilltoborder($im,$ret_x01 - 1,$ret_y01 +11,$black,$cor02);
imagestring($im,3,$ret_x01 + 20, $ret_y01, $slice[1] . "% $strFree",$black);
imagestring($im,3,$ret_x01-60,$ret_y01 + 200,"$strSpcAll : " . round(($_GET['total'])/1024,0) ." $byteUnits[1]",$black);
imagestring($im,3,$ret_x01-60,$ret_y01 + 215,"$strUsed : " . round(($_GET['usado'])/1024,0) ." $byteUnits[1]",$black);
imagestring($im,3,$ret_x01-60,$ret_y01 + 230,"$strFree : " . round(($_GET['total'] - $_GET['usado'])/1024,0) ." $byteUnits[1]",$black);

$ret_x01=300;
$ret_y01=70;
imagefilledrectangle($im, $ret_x01,$ret_y01,$ret_x01 + 15 ,$ret_y01 + 15,$cinza);
imagerectangle($im,$ret_x01 - 3 ,$ret_y01 - 3,$ret_x01 + 12 ,$ret_y01 +12,$black);
imagefilltoborder($im,$ret_x01 - 1,$ret_y01 +11,$black,$cor03);
imagestring($im,3,$ret_x01 + 20, $ret_y01, $slice[2] . "% $strUsed",$black);

//sombra no circulo
ImageFillToBorder($im, 150 + 72, 150 + 72, $black, $cinza);
ImageArc($im, 153, 153, $diameter, $diameter,315,135,$cinza);
// write string
ImageString($im, 5, 80, 10, "$strSpcUsed", $black);

// output to browser
ImagePNG($im);
?>

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