Question

Could anybody explain why I am able to display a php graph when I use the code below only in a php file but, when in a php file with additional code, control characters print instead of the graph?

function display_graph() {
//begin graph
//  # ------- 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);
            imagepng($img);
}
?>

Answer 1

In order to work your additional code should not send data on the standard output.
Even a single space could corrupt your image. If the image is corrupted it not fit with the Content-Type and the browser not display the image but its binary code.

Check things like , print , echo …, in your additonal code.