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Plugin API/Filter Reference/query vars

Allows query vars to be added, removed, or changed changed prior to executing the query.

Example 

function themeslug_query_vars( $qvars ) {
array_push( $qvars, 'custom_query_var' );
return $qvars;

}
add_filter( 'query_vars', 'themeslug_query_vars' , 10, 1 );
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