php - Using $_GET with clean URL's

I know this has been asked many a time, but i cannot find the answers I'm looking for.

Basic bit of URL rewriting but first time doing it.

example url:

domain.com/viewbuild.php?id=25
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Desired:

domain.com/viewbuild/25
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Attempted htaccess code:

RewriteRule ^viewbuild/([a-zA-Z0-9]+)/$ viewbuild.php?id=$1 [L,QSA]
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However, I am trying to use:

$id = $_GET['id'];
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But i am getting errors with this for undefined index.

I thought because i am rewriting the URL that GET variables would still work?

Thanks. Craig.

Edit as requested, full .htaccess.

RewriteEngine on
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^viewbuild/([a-zA-Z0-9]+)/?$ /viewbuild.php?id=$1 [L,QSA]
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule !.*\.php$ %{REQUEST_FILENAME}.php [L,QSA]
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