It is not recommended to use query_posts. But this is not the problem, Problem is you are using PHP variable in single quotes. 'tag=$tag'
Consider these examples:
query_posts('tag=php');
//OR
query_posts("tag=$tag"); //$tag should output: php
Using WP_query //Recommended
$query = new WP_Query(array( 'tag' => 'php' ));
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