如果文本包含某个单词，如何从文本文件中删除字符串

I am trying to figure out how to remove everything between and including in a text file.

So far I have this that removes a whole urlset from a file.

$myFile = "/here/it/is/sitemap.xml"; 

$stringdata = file_get_contents($myFile);

$stringdata = preg_replace('#(<urlset.*?>).*?(</urlset>)#', '$1$2', $stringdata);

$stringdata = str_replace('<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:video="http://www.google.com/schemas/sitemap-video/1.1"></urlset>', '', $stringdata);

$fh = fopen($myFile, 'w') or die("can't open file");

fwrite($fh, $stringdata);
fclose($fh);

return;

The problem is I have several urlsets in the text file but I only want one of the urlsets to be removed, which is the one that matches a certain variable, so something like this (which doesn't work) :

$stringdata = preg_replace('#(<urlset.*?>).*? . $thisvariableisintheurlset . (</urlset>)#', '$1$2', $stringdata);

Does anyone know what I need to add to this to remove the targeted urlset, or even a better way if this looks like a bad way to attack the problem?

Thanks

try to use http://php.net/manual/en/function.preg-quote.php

$stringdata = preg_replace('#(<urlset.*?>).*?' . preg_quote($thisvariableisintheurlset, '#') . '.*?(</urlset>)#', '$1$2', $stringdata);

also make sure that the things before and after the quoted variable are matched (I added .*?, but that might be not correct for your case)