I'm creating some images from WP posts using HTML5 Canvas and saving them to my uploads folder. I'm then sharing them to Facebook, which works fine, but to share to Twitter I need to update the Twitter Meta tags with the image URL's. I'm aware I can't do this with JS and need to do it server side, but I can't work out how to to this...

I'm using the following function in functions,php

add_action('wp_ajax_nopriv_canvasUpload', 'canvasUpload');
add_action('wp_ajax_canvasUpload', 'canvasUpload');

function canvasUpload(){
$uploads = wp_upload_dir();
$img = $_POST['uploadImage'];
$img = str_replace('data:image/png;base64,', '', $img);
$img = str_replace(' ', '+', $img);
$data = base64_decode($img);


global $file;
$uploads = wp_upload_dir(); 
$file = $uploads['path'].'/'. uniqid() . '.png';
echo $file;

return 
$r = file_put_contents($file, $data);
echo $r ? $file : 'Error saving file';
}




function add_twitter_cards() {
    if(is_single()) {
    $tc_url    = get_permalink();
    $tc_title  = get_the_title();
    //$tc_description   = get_the_excerpt();

    global $post;
    global $file;

    $tc_image = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' );
    $tc_image_thumb  = $tc_image[0];
    $tc_author   = str_replace('@', '', get_the_author_meta('twitter'));
?>
    <meta name="twitter:card" value="summary" />
    <meta name="twitter:site" value="@elegantthemes" />
    <meta name="twitter:title" value="<?php echo $tc_title; ?>" />
    <meta name="twitter:description" value="" />
    <meta name="twitter:url" value="<?php echo $tc_url; ?>" />
<?php global $file; ?>
      <meta name="twitter:image" value="<?php echo $file; ?>" /> 
        <?php if($tc_author) { ?>
        <meta name="twitter:creator" value="@<?php echo $tc_author; ?>" /> 
        <?php

    }
    }
} 

add_action('wp_head', 'add_twitter_cards');

and an AJAX call which currently shares the images to Facebook:

    $(document).on('click','.twitter',function(e){

    $.ajax({
    type: "POST",
    url: ajaxurl,
    data: {
    action: 'canvasUpload',
    uploadImage: data
    }
    }).done(function(o) {
      var url = o.split('uploads');
      var fileURL =  $("#fileURL").html(url[1]);
      var imgURL = $('#imgURL').html();
      var finalURL = imgURL + $("#fileURL").html().replace("png0", "png");;
      
 window.open(
          '.php?u='+ finalURL +'',
          "popupFacebook",
          "width=650, height=270, resizable=0, toolbar=0, menubar=0, status=0, location=0, scrollbars=0"
        );
       });
    
        return false;
    });
    });

Any help much appreciated,