ajax requests for postbit

[Wedframe]

Hello! I try to make custom plugin, and try to figure out, how I can use ajax requests for suitable changes in post content.
On hook bbcode_create:

PHP Code:

/// Get code HTML 
global $vbulletin, $post; 

if (!function_exists ('handle_geshi_code')) 
{ 
  function handle_geshi_code($code, $lang) 
  { 
    require_once(DIR . '/geshi.php'); 
    $codefind1 = array('<br>','<br />'); 
    $codereplace1 = array('',''); 
    $codefind2 = array('&gt;','&lt;','&quot;','&amp;','[',']',); 
    $codereplace2 = array('>','<','"','&','[',']',); 
    $code = rtrim(str_replace($codefind1, $codereplace1, $code)); 
    $code = str_replace($codefind2, $codereplace2, $code); 
    $geshi = new GeSHi($code, $lang); 
    $geshi->set_overall_style('font-size: 12px'); 
    $geshi->enable_line_numbers(GESHI_NORMAL_LINE_NUMBERS); 
    $code = $geshi->parse_code(); 
    $code = preg_replace('/&amp;#([0-9]+);/', '&#$1;', $code); 
    $code = str_replace(array('[', ']'), array('[', ']'), $code); 
    return $code; 
  } 
} 

if ($this->is_wysiwyg())  
  {  
        $this->unparsed_tags = array( 
        'phps',
        'ahk',
    'au3',
    'avs',
        'diff',
        'xml'
  ); 
  } 


and on bbcode_fetch_tags

PHP Code:

$tag_list['no_option']['phps'] = array ( 
    'callback' => 'handle_external', 
    'external_callback' => 'handle_geshi_php', 
    'disable_wordwrap' => true,   
    'strip_empty' => true  
); 

if (!function_exists ('handle_geshi_php')) 
{ 
  function handle_geshi_php(&$theobj, &$value, &$option) 
  { 
    $code = handle_geshi_code($value,'php'); 
    return '<div class="bbcode_container"><div id="geshipars"><script type="text/javascript" src="/geshi/scripts/geshi.js"><span onclick="return myfn(' . $post['postid'] . ');">Click</span></a></div><div style="max-height: 300px;" class="bbcode_code">'.$code.'</div></div>'; 
  } 
} 


with js:

function myfn(postid)
{
    var sUrl = 'ajax_geshi.php';
}

but in debugger I have:
postid = undefined
--------------------------------------
So... whats right syntax for this operation?
And next... how I can resolve bbcode-parsed text from current post?
Thanks!