在php中创建条形图的问题

我使用此代码尝试在php中创建条形图

    <?
        # ------- The graph values in the form of associative array
        $values=array(
        "Jan" => 110,
        "Feb" => 130,
        "Mar" => 215,
        "Apr" => 81,
        "May" => 310,
        "Jun" => 110,
        "Jul" => 190,
        "Aug" => 175,
        "Sep" => 390,
        "Oct" => 286,
        "Nov" => 150,
        "Dec" => 196
    );

    $img_width=450;
    $img_height=300; 
    $margins=20;

    # ---- Find the size of graph by substracting the size of borders
    $graph_width=$img_width - $margins * 2;
    $graph_height=$img_height - $margins * 2; 
    $img=imagecreate($img_width,$img_height);

    $bar_width=20;
    $total_bars=count($values);
    $gap= ($graph_width- $total_bars * $bar_width ) / ($total_bars +1);

    # -------  Define Colors ----------------
    $bar_color=imagecolorallocate($img,0,64,128);
    $background_color=imagecolorallocate($img,240,240,255);
    $border_color=imagecolorallocate($img,200,200,200);
    $line_color=imagecolorallocate($img,220,220,220);
    # ------ Create the border around the graph ------
    imagefilledrectangle($img,1,1,$img_width-2,$img_height-2,$border_color);
    imagefilledrectangle($img,$margins,$margins,$img_width-1-$margins,$img_height-1-$margins,$background_color);

    # ------- Max value is required to adjust the scale -------
    $max_value=max($values);
    $ratio= $graph_height/$max_value;

    # -------- Create scale and draw horizontal lines  --------
    $horizontal_lines=20;
    $horizontal_gap=$graph_height/$horizontal_lines;
    for($i=1;$i<=$horizontal_lines;$i++){
        $y=$img_height - $margins - $horizontal_gap * $i ;
        imageline($img,$margins,$y,$img_width-$margins,$y,$line_color);
        $v=intval($horizontal_gap * $i /$ratio);
        imagestring($img,0,5,$y-5,$v,$bar_color);
    }

    # ----------- Draw the bars here ------
    for($i=0;$i< $total_bars; $i++){ 
        # ------ Extract key and value pair from the current pointer position
        list($key,$value)=each($values); 
        $x1= $margins + $gap + $i * ($gap+$bar_width) ;
        $x2= $x1 + $bar_width; 
        $y1=$margins +$graph_height- intval($value * $ratio) ;
        $y2=$img_height-$margins;
        imagestring($img,0,$x1+3,$y1-10,$value,$bar_color);
        imagestring($img,0,$x1+3,$img_height-15,$key,$bar_color);       
        imagefilledrectangle($img,$x1,$y1,$x2,$y2,$bar_color);
    }
    //header("Content-type:image/png");
    imagepng($img);
   ?>

但是，当在apache上运行时，我在浏览器上得到的只是这个。我不知道为什么会发生这种情况：

Warning: Cannot modify header information - headers already sent by (output started at F:'AppServ'www'bargr.php:3) in F:'AppServ'www'bargr.php on line 79
‰PNG  IHDRÂ,-8[$PLTE@€ððÿÈÈÈÜÜÜOS› zIDATxœíMŽò8†Ê¦§¥dßÊÊòö;@ƒDö …Íœ±Š| Ë+˧˜Œô]e$¶hz’ØI

还有一些像这样奇怪的数字。我的php版本有问题吗？有人能帮我吗？