Description:
------------
IN MY CODE , there is an empty code block before the real useful code block , the empty code block cause no img output in my server 

Reproduce code:
---------------
<?php
///THIS BLOCK OF CODE CAN CAUSE NO IMG OUTPUT

//  ???????±???һ??
//   created by linuxtyh
//   08.12.2006 6:35:40 PM
//
?>
 <?php
 error_reporting(0);
$width=700;
$height=700;
$image = imagecreate($width, $height);
$bg = imagecolorallocate($image, 200, 200, 200);
$textcolor = imagecolorallocate($image, 0, 0, 255);
$linecolor = imagecolorallocate($image, 20, 100, 200);

 
if(!isset($_GET["function"])||$_GET["function"]=="")
{
      imagestring($image, 5, 0, 0, "no function found.", $textcolor);
      //echo("no func");
}
else
{
    $function= $_GET["function"];
    imagestring($image, 5, 0, 0, "function: y=$function ", $textcolor);
//echo("found func");



$amp=30;   //amplify
   if( isset($_GET["amplify"]))
   {    $amp= $_GET["amplify"];
   }
$dx=0;  //offset
$dy=0;   //
$start=-$width/2;
$end=$width/2;
      $function=str_replace ("x","((\$x)/$amp-$dx)" ,$function);
       $function="\$y=$height/2-$amp*($function+$dy);" ;//+";";
     $x=$start;
     $y;
     
   try{
             eval($function);
       }catch(Exception $error)
       {
           	$y=1e20;
       }
      $oldx=$x;$oldy= $y   ;
   imageline($image,  0 , $height/2,   $width   ,  $height/2 , $linecolor);
      imageline($image,  $width /2 , 0,   $width /2  ,  $height  , $linecolor);
     $ox=$width /2;$oy=$height/2;
     for($i=0;$i<$width/2;$i+=1*$amp)
     {
         imageline($image,  $width /2+$i , $height/2,  $width /2+ $i   ,  $height/2-2 , $linecolor);
         imagestring($image, 1, $width /2+$i , $height/2+2 , ""+$i/$amp , $textcolor );
         imageline($image,  $width /2-$i , $height/2,  $width /2- $i   ,  $height/2-2 , $linecolor);
         imagestring($image, 1, $width /2-$i , $height/2+2 , ""+(-$i)/$amp , $textcolor );
     }
     for($i=$amp;$i<$height/2;$i+=1*$amp)
     {
         imageline($image,  $width /2  , $height/2+$i,  $width /2+ 2   ,  $height/2+$i , $linecolor);
         imagestring($image, 1,$width /2 +4 , $height/2+$i, ""+(-$i)/$amp , $textcolor );
         imageline($image,  $width /2  , $height/2-$i,  $width /2+ 2   ,  $height/2-$i , $linecolor);
         imagestring($image, 1,$width /2 +4 , $height/2-$i, ""+($i)/$amp , $textcolor );
     }

     for($x=$start; $x<=$end; $x++)
    {
              try{
             eval($function);
       }catch(Exception $error)
       {
       $y=1e20;
       }
           imageline($image, $oldx+$width/2 , $oldy, $x+$width/2  , $y , $linecolor);
              $oldx=$x;
              $oldy=$y;
     }
 } 
header("Content-type: image/png");
imagepng($image);
  imagedestroy($image);
?>


Expected result:
----------------
a  math function  image

Actual result:
--------------
no image output  but an image of red cross