This function adds another name/value pair to the URL rewrite mechanism. The name and value will be added to URLs (as GET parameter) and forms (as hidden input fields) the same way as the session ID when transparent URL rewriting is enabled with session.use_trans_sid. Please note that absolute URLs (http://example.com/..) aren’t rewritten.
Example #1 output_add_rewrite_var() example
<?php     
output_add_rewrite_var('var', 'value');      
// some links      
echo '<a href="file.php">link</a>      
<a href="http://example.com">link2</a>';      
// a form      
echo '<form action="script.php" method="post">      
<input type="text" name="var2" />      
</form>';      
print_r(ob_list_handlers());      
?>
The above example will output:
<a href="file.php?var=value">link</a>
<a href="http://example.com">link2</a>
<form action="script.php" method="post">
<input type="hidden" name="var" value="value" />
<input type="text" name="var2" />
</form>
Array
(
    [0] => URL-Rewriter
)
http://php.net/manual/en/function.output-add-rewrite-var.php