passing value form URL to form

Can you please help me. I need to pass a value from a lin to my form and all the examples I'm finding on the forum thus far don't seem to work.
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They all seem to involve some variation of this code, which when I add to a custom code block in chronforms6 just seems to throw an error (below)
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<?php
if ( !$mainframe->isSite()) {return;}
$var = JRequest::getInt('session', '0', 'get');
?>
​
ERROR = 0 Call to a member function isSite() on null
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Here is an example of the link with the value I want to pass, is that correct? I want that session ID to pass to the form somehow. I need to add that session ID to the name of the signature that is saved once the form submits. Can you please help?
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http://XXXXXX/index.php?option=com_content&view=article&id=20&session=544

{data:name_of_url_parameter}
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And you're getting that error because you haven't set $mainframe. You might need to have a closer look at the code you're using to see what you're missing.

Hi jformicola,
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I believe that CF adds the URL data to the form data for you. Please add a Debugger action so you can see what is available in the On Load event.
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Bob

Sure enough it was there, thanks. So I didn't need the code to get it there (which is good because I didn't understand healyhatman's comment about the mainframe haha) Thanks guys!
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This was the PHP code I used to get the value into the signature if it's helpful for others...
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<?php
$data_pieces = explode(",", $this->data[signature8]);
$encoded_image = $data_pieces[1];
$decoded_image = base64_decode($encoded_image);

// set the folder where the images will be saved
$folder = 'images/signatures/';
$path = JPATH_SITE.'/'.$folder;
$url = JURI::root().$folder;
// sets a file name using a date-time prefix to make sure it is unique
$sig_name = $this->data("session").'_'.$this->data("name").'.png';
file_put_contents( $path.$sig_name, $decoded_image);

// add the file info into the form data
return array(
'url' => $url.$sig_name,
'path' => $path.$sig_name
);
?>