Variables & PHP script called from <img src=

Ldo

My first post here so please be gentle 🙂

I have just spent the last 4 hours trying to find an answer to this and finally conceded to asking here as I cannot even find anyone else having a similar problem. This may be a long post...

Background: I am generating dynamic images using variables stored in a session (in a MySQL D😎. These variables are placed as they get set (Through $POST, $GET) in the session and retrieved by the dynamic image creation script when called.

This image script is called from within an HTML image tag as follows <img src="imagescript.php?opt1=123&opt2=abc">

All is working well up to this point. Now comes the problem... how do I get any values returned from this script? This script allows user to place an image over an existing image (like a watermark) and they can specify the size of the watermark. What I am trying to do is get the image creation script to send a flag back to the calling script if the watermark they are placing is too big for the image. This is proving to be harder than would be expected.

It seems that PHP 5 (don't know about older versions) does not pass on any global variables to a script called in this way. What I mean by that is in the image script I am unable to get or set any global variables which are accessible from the calling script. My PHP instance does have Globals set off, but I have set it on as a test and this makes no difference.

I first noticed this when I was unable to read the session data from the image script. I worked around this by reconnecting to the DB and unserializing the session data manually to get to the variables.

Am I missing something here or is this just a problem with my set up?

I have tried writing the flag to the DB and checking for the flag on the calling script but the flag only seems to be present after the second click on a page (one click behind).

Any help in tis regard would be appreciated as I am pulling out my hair :mad:

ClarkF1

Some code would help us solve your problem.

Please surround it with [ php ] [/ php] tags (remove the spaces) so we get the syntax highlighting

Ldo

Thanks ClarkF1.

The code has evolved to a bit of a monster so pasting it all here would be a nightmare. To simplify things, lets say the main script is called index.php.

When index.php runs the user inputs values through various means (POST and GET). These are set into their session. The user then clicks through to a preview page - lets call it preview.php. Preview.php contains mostly HTML and an HTML image tag which calls the main dynamic image creation script (lets call this image.php). Image.php is called from preview.php in an IMG tag like this:

(not the actual code so don't look for errors ;-))

This is how I am calling the image creation script (in preview.php):

<img src="/image.php?pid=<?php echo $image_id;?>&amp;sid=
<?php echo session_id();?>&amp;s=p&amp;csq=
<?php echo rand();?>" alt="<?php echo $info;?> Preview - <?php echo $image_name;?>"/>
<?php
//Attempt 1: Global variable. Does not work
global $oversize_flag;
if ($oversize_flag) echo 'Image oversize!';

//Attempt 2: Write flag to database
$sql = "SELECT oversize_flag FROM oversize_temp 
WHERE sesskey = '".session_id()."' 
AND image_id = '".$image_id."'";
$oversize_flag_result = mysql_query($sql);
$oversize_flag_array = mysql_fetch_array($oversize_flag_result,MYSQL_ASSOC);
$oversize = $oversize_flag_array['oversize_flag'];
if ($oversize == '1'){
	echo 'Image over maximum allowed size!!';
}
?>

image.php pulls all the values entered earlier (like colour, size, watermark file name, etc) from the unserialized session data which it is pulling out of the database and creates the image (using the GD libraries). This all works well. The problem is that I want to get information back from this process in a realtime fashion. If the image created by image.php is (for example) larger than allowed I would like to be able to display a warning message on preview.php right after calling image.php.

As mentioned earlier I am passing on the session ID as a GET variable as it is not being done by PHP (which is to be expected as it is not being included in preview.php in a standard require or include).

The main problem with trying to get anything returned by image.php is that because it is being called in an HTML IMG tag PHP is not "passing on" any of the super globals or global variables which would be my only real way of retrieving realtime feedback from the image creation process.

image.php code looks like this (summarised - not the real code):

<?php
header("Content-type: image/png");
//Connect to DB
//get session data
//unserialise session array
//set local variables according to values from session data
//create image according to variable
$source_image = imagecreatefrompng($image_path);
$watermark = imagecreatefromjpeg($watermark_path);
//get the sizes of both original images
$watermark_width = imagesx($watermark);
$watermark_height = imagesy($watermark);
$source_image_width = imagesx($source_image);
$source_image_height = imagesy($source_image);
$image_out = imagecreatetruecolor($source_image_width,$source_image_height);
imagecopy($image_out,$watermark,0,0,0,0,$watermark_width,$watermark_height);
//get size of output image
$output_width = imagesx($image_out);
$output_height = imagesy($image_out);

if ($output_width > $maximum_width || $output_height > $maximum_height){
	//set the oversize flag to warn the user
	//Attempt 1: global variable flag
	global $oversize_flag; // DOES not work as no global variables ever
                        //reach the calling script
	$oversize_flag = TRUE;	

//Attempt 2: Database flag - Sort of works but flag is one click behind. 
              //What that means is that as the image is generated no warning is 
             //given on the calling page (preview.php) until the page is refreshed 
             //again. Not sure why this happens...
$sql = "INSERT INTO oversize_temp (sesskey,image_id,oversize_flag)
 values('".addslashes($_GET['sid'])."','".(int)$_GET['pid']."','1')
	";
	mysql_query($sql);
}
?>

I guess my question really is "is there a better way of doing this?" or "is there a workaround for what I am trying to do?". Is this a bug with PHP or is it handling the variables the way it should? I always thought a global variable really was GLOBAL? I guess not...

ClarkF1

Using global $oversize_flag won't work in this case. You need to store the value in a SESSION variable or through a query string and use $oversize_flag = $_GET['oversize_flag'];

global is used for variable scope e.g. making variables outside a function accessible inside without having to pass them as function parameters

<?php
$a = 1; /* global scope */

function Test()
{
   echo $a; /* reference to local scope variable */
}

Test();
?>

This won't print anything because the $a inside the Test function doesn't have access to the value outside the function because it's scope is local i.e. only within the function

<?php
$a = 1;
$b = 2;

function Sum()
{
   global $a, $b;

   $b = $a + $b;
}

Sum();
echo $b;
?>

The above script will output "3". By declaring $a and $b global within the function, all references to either variable will refer to the global version. There is no limit to the number of global variables that can be manipulated by a function.

You can't pass the values of variables from one php file to another just by using global

Ldo

Thanks ClarkF1. That is predicament I find myself in. I cannot call the image file in any other way if I want the browser to output the image correctly. I would have to recreate the session in the image file as PHP is not passing on the session information with trans_sid as it is seen as outside the "scope" of the main script execution.

I guess there is no easy way to solve this one?

Ldo

I did some more testing. It seems that the reason my database flag workaround does not work the way I expected it to (always one click behind) is because php is not waiting for the image generation script to complete.

I added a sleep command to the image creation script (image.php) and the rest of the page loads around it. What that means to me is that my flag is not in the database yet by the time the flag check (called after <img> tag) checks and is only picked up on the next page load. Is there any way for me to get PHP to wait with the execution of the preview.php script until image.php completes its code?

<edit>Even stranger is that if I add a 30 second sleep after calling image.php in the IMG tag and take a look at the database my flag does not appear until the execution of preview.php completes 😕 Why would this be happening?</edit>

Ldo

OK. I have managed to create a workaround that does what I want by moving most of the image creation code into a function and saving it's output to a blob. The I retrive the blob by another script which outputs the blob along with the correct header. I am not a fan of storing images as blobs in a database but it seems that this may be the only option for me in this case.

This raises another debate/question that I would really like answered...
I am running PHP with session data stored in a MySQL database.

The question I have now is if I am storing image data in the database (against my better judgement) would it be better to store this data inside the session or in a seperate table?

At least if I store it in the session I don't have to worry about cleaning up after each page (and don't have to worry about multiple access) but I am worried that adding too much to the session table will slow everything down?? There will only ever be at most 3 images stored this way for each session/user, and each image will never be over 80KB in size.

Which option is best? Advice please 🙂

Moderators: I am going to leave this thread open for now as I would still be interested if someone could explain the behaviour I was experiencing when calling the script inside an img tag.