Trouble passing variable from page to page using $_SESSION php - php

Question

I am doing an assignment for school where we are to make two php files. First file invokes session, generates a random 5 char string, and saves the string to the session array. The second script generates an image, and takes the string from the first file and incorporates it over the image to make a captcha.
I am having an issue passing the value to the second script. The session variable 'captcha_string' is fully visible in the first script, but is not passed to the second page. I am brand new at this, and frustrated. My understanding is, that as long as I start session, the entire $_SESSION array should be available. When I run the first script, I get a broken image tag, not the captcha i am hoping for. Hope this clears up my problem.
This is what I have done for the first file:
<?php
session_start();
$possible_chars = array_merge(range('A','Z'),range('0','9'));
shuffle($possible_chars);
$string = substr(implode($possible_chars),0,5);
$_SESSION['captcha_string']=$string;
?>
<img src="captcha_generator.php" alt="Weinerdog!" />
and this is the bit from the second file where I try to grab the $string (captcha_string), which is named "captcha_generator.php:
<?php
session_start();
putenv('GDFONTPATH=' . realpath('.'));
header("Content-type: image/png");
//import string for the captcha from $_SESSION
$string = $_SESSION['captcha_string'];
// Build an image resource using an existing image as a starting point.
$backgroundimage = "captcha_wiener.jpg";
$im=imagecreatefromjpeg($backgroundimage);
$colour = imagecolorallocate($im, rand(0,255), rand(0,255), rand(0,255));
// Output the string of characters using a true type font.
// Above we set the font path to the current directory, this
// means that arial.ttf font file must be in this directory.
$font = 'arial.ttf';
$angle = rand(-5,5);
imagettftext($im, 120, $angle, 50, 250, $colour, $font, $string);
// Draw some annoying lines across the image.
imagesetthickness($im, 10);
for ($i = 0; $i <3; $i++) {
imageline($im, rand(100,50), rand(150,200), rand(450,550), rand(200,250), $colour);
}
// Output the image as a PNG and the free the used memory.
imagejpeg($im);
imagedestroy($im);
?>
This is, of course, strictly an exercise to make sure we can pass values using session. There is no problem with the rest of the code making the captcha, it has been tested and works.

Answer 1

You're echo-ing some values with a content-type set to image/png, hence either you'll have the error of headers already sent, or, if the text wasn't sent yet (because cached by PHP), you'll have a broken image and you won't be able to see the text.
Don't worry, it has happened to everyone including me :-)