Using str_replace() to modify query, is it possible?

[Antivirus]

In newattachment.php i want to modify the following query:
[SQL]
SELECT dateline, filename, filesize, attachmentid, IF(thumbnail_filesize > 0, 1, 0) AS hasthumbnail
FROM " . TABLE_PREFIX . "attachment
WHERE postid = $postid
ORDER BY attachmentid
[/SQL]

so it becomes this:
[SQL]
SELECT dateline, filename, filesize, attachmentid, IF(thumbnail_filesize > 0, 1, 0) AS hasthumbnail
FROM " . TABLE_PREFIX . "attachment
WHERE scst_usedin = 'fbid' AND scst_itemid = '" . $feedback['fbid'] . "'
ORDER BY attachmentid
[/SQL]

The problem is that there's no hook in newattachment.php which allows me to alter the query, so I'm thinking it's possible to place a plugin at hook: GLOBAL_START like this:

PHP Code:

if (THIS_SCRIPT == 'newattachment' AND $_REQUEST['type'] == 'fb') 
{ 
    str_replace(
        ' WHERE postid = $postid ',
        ' WHERE scst_usedin = \'fbid\' AND scst_itemid = \'\" . $feedback['fbid'] . \"\' ',
        $currentattaches2
    );
} 


I know this won't work, but is there some similar function in PHP which tells the engine "when you get to a certain string of code, use one thing instead of what was originally there"?

thanks!