My thumbnail function is't working properly - php

Question

When this code runs the browser screen turns black with a grey and white checkered square in the middle when it is suppose to display a thumbnail within a table cell on a form.
function createThumb( $imageUrl, $thumbWidth )
{
// load image and get image size
$img = imagecreatefromjpeg( $imageUrl );
$width = imagesx( $img );
$height = imagesy( $img );
// calculate thumbnail size
$new_width = $thumbWidth;
$new_height = floor( $height * ( $thumbWidth / $width ) );
// create a new temporary image
$tmp_img = imagecreatetruecolor( $new_width, $new_height );
// copy and resize old image into new image
imagecopyresized( $tmp_img, $img, 0, 0, 0, 0, $new_width, $new_height, $width, $height );
// display thumbnail
header("Content-type:image/jpeg");
imagejpeg( $tmp_img,$imageUrl,80);
};
And the line that calls the code is
<td >
<?php echo createThumb("CoffeeReg.jpg",75)?>
</td>
I haven't found any of the normal syntax errors, and the code even comes back clean when ran through a syntax checker so that leads me to believe either its a problem with my logic, I'm using the built in functions incorrectly or I have messed up something in the way I am trying to display the new thumbnail only with my lack of exp I don't see where I went wrong. I need to know what I did wrong so I can learn from this mistake.

Answer 1

Okay so after several hours debugging the code that Majid had provided when trying to help me understand where I went wrong with my code I found that the only thing that was needed to make the code he was using work was to add these two lines of code to the end of the secondary php file as shown here...
<?php
$imageUrl = $_GET['file_name'];
$thumbWidth = $_GET['thumb_width'];
// load image and get image size
$mainImage = imagecreatefromjpeg( $imageUrl );
$mainwidth = imagesx( $mainImage );
$mainheight = imagesy( $mainImage );
// calculate thumbnail size
$thumbWidth = intval($mainwidth/4);
$thumbHeight = intval($mainheight/4);;
// create a new temporary image
$tmp_img = imagecreatetruecolor( $thumbWidth, $thumbHeight );
// copy and resize old image into new image
imagecopyresampled( $tmp_img, $mainImage, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $mainwidth, $mainheight );
// display thumbnail
header("Content-type:image/jpeg");
//Also had to remove the second and third argument from this function
imagejpeg( $tmp_img);
//these following two lines here is what was missing
imagedestroy(tmp_img);
imagedestroy(mainImage);
?>
So I guess that one must clear out the image variables of the thumbnail code or it will break it when it runs. Not sure why removing the two arguments from the imagejpeg function was also needed but it was mentioned in my textbook so i tried it, now with these changes this code works. Thank you Majid for all your help and If your post was still here you would be getting full credit with an up vote and best answer.