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This function can be used to make WordPress aware of custom querystring variables. Generally, it's used in combination with add_rewrite_rule() to create rewrite rules for pages with custom templates.
If you use this function to declare a rewrite tag that already exists, the existing tag will be overwritten.
This function must be called on init or earlier.
The following will successfully register a tag called 'film_title':
This is particularly important when you are using rewrites with custom page templates.
With a rewrite tag defined, you can now retrieve the value of your rewritten querystring variables using WordPress's $wp_query variable. To get the value of the above tag out of a rewrite, you could use the following in your page template:
Note that using $_GET on a rewritten URL will not work, even if the rewrite includes the querystring variables. You must use $wp_query.
add_rewrite_tag() is located in