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Editorial Team
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Asked: 2026-05-26T07:43:51+00:00

I have simple question – why my *.png picture, created by PHP GD, is

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I have simple question – why my *.png picture, created by PHP GD, is not displayed properly on the web page?
My PHP code is:

back.php

    <?php
    $height = 200;
    $width = 600;
    $jVar = $_POST['jPacket']; //velue from jQuery script

    $im = imagecreatetruecolor($width, $height);
    $white = imagecolorallocate($im, 255, 255, 255);
    $black = imagecolorallocate($im, 0, 0, 0);
    imagefill($im, 0, 0, $white);
//line_1
    imageline($im, 10, 0, 10, $height, $black);
//line_2
    imageline($im, $jVar, ($height/2), $height-$jVar, ($height/2), $black);

    header('Content-type: image/png');
    imagepng($im);
    imagedestroy($im);
?>

…and simple jQuery AJAX post is here:

script.js

var graphBack = $('<img id="backGraph" src="back.php" alt="backGraph"/>');

$('.value').keyup(function() {
      $.post(
          "back.php", //send the POST here
      {
           jPacket: $('.value').val() //value from <input>
      }, function() {
           $('#backGraph').remove(); //remove old picture
           graphBack.insertAfter($('#sipGraph p')); //put the picture here
      });

The picture is displayed properly but not everything is visible. The ‘line_1’ is visible because all elements in imageline(...) are static but the ‘line_2’ is not displayed because 'imageline(..., $jVar, ...) has value that is send by $_POST['jPacket'] from scipt.js.
Do I properly show the IMG in graphBack.insertAfter($('#sipGraph p'));?

When I perform above scanario but data is sent not from jQuery but ordinary HTML FORM everything is ok…

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1 Answer

  1. Editorial Team

    The post you are sending is not what is being shown on the page. When you attach var graphBack = $('<img id="backGraph" src="back.php" alt="backGraph"/>'); to the page, you are creating a second, separate request with no post – essentially loading an image on the page. Back is being called twice essentially, and the time with POST is not the response that you are displaying.

    I would instead simply attach a generated image tag along the lines of $('<img id="backGraph" src="back.php?jPacket='+ MYAWESOMEJPACKET +'" alt="backGraph"/>'); and replace the current one – further updating the script to use GET.

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