<?php
header ("Content-type: image/jpeg");
if($_GET['term']){
$term=$_GET['term'];
}
else {
$term="\$x";
}
$im = ImageCreate(1000,1000);
if($im) {
    $bg_color = ImageColorAllocate ($im, 240, 240, 240);
    $fg_color = ImageColorAllocate ($im, 0, 0, 0);
    $fg_color2 = ImageColorAllocate ($im, 200, 200, 200);
    for($a=25;$a<=1000;$a+=25){
    imageline($im,$a,0,$a,1000,$fg_color2);
    }
    for($b=25;$b<=1000;$b+=25){
    imageline($im,0,$b,1000,$b,$fg_color2);
    }
    imageline($im,0,500,1000,500,$fg_color);
    imageline($im,500,0,500,1000,$fg_color);
    imageline($im,550,504,550,496,$fg_color);
    imageline($im,450,504,450,496,$fg_color);
    imageline($im,496,550,504,550,$fg_color);
    imageline($im,496,450,504,450,$fg_color);
   for($x=-10;$x<=10;$x=$x+0.02){
   $pixel_x=500+50*$x;
   if($x==0){
   $pixel_y=0;
   } else {
   eval("\$pixel_y=500-round(50*(" . $term . "),2);");
   }
   imagesetpixel($im,$pixel_x,$pixel_y,$fg_color);
   }
   imagejpeg($im);
}
?>

<?php
if(isset($_REQUEST['term'])){
?>
Neuen Term angeben
[img]mathfunction.php?term=<?php echo rawurlencode($_REQUEST['term']) ?>[/img]
<?php
} else {
?>
<form action="math.php" method="get">
Funktionsterm: <input type="text" name="term">

<input type="submit" name="los" value="Los!">
</form>
<?php
}
?>