PHP PEAR CLASS (SearchReplace) Need to replace variable text between " and "

I have a simple application based on the php pear class SearchReplace:

// Define result of Activate click
    if (isset($_POST['action']) and $_POST['action'] == 'Activate')
    include "$docRoot/includes/pear/SearchReplace.php" ;
    $files = array( "$docRoot/promotions/index.php" ) ;
    $snr = new File_SearchReplace( '$promoChange = "";', '$promoChange = "'.$currentYear.'/'.$currentPromotion.'";', $files) ;
    $snr -> doSearch() ;

Notice the string that it is configured to search for:

$promoChange = "";

What I need to do, is replace this string, when there is a varying value assigned to the variable $promoChange So basically, what I am trying to replace would look something like this:

$promoChange = "2011/sep-oct";

I would need something along the lines of:

$promoChange = "%";

What would be the correct way of doing this?

If anyone has any input in this matter, it would be appreciated greatly,

Thank You!