Use of union in mysql-thinkphp

Mysqlthinkphpphpunion

Code
$result 4= $activity->where ("topic= '%s ' and city= '%s '", "Top", I (' get.city '))->field (' Createtime,title, Readcount,id,belong ')->table (' gq_activity ')->union (' SELECT createtime,title,readcount,id,belong from Gq_ Preferential where topic= "top" ')->union (' SELECT createtime,title,readcount,id,belong from Gq_multimedia WHERE topic= "Top")->union (' SELECT createtime,title,readcount,id,belong from Gq_travel WHERE topic= ' top ')->select ( );
Problems
How to use variables in the Where in the Union (' select Createtime,title,readcount,id,belong from gq_preferential where topic= ' top ') to filter out the data. Because if you use Union (' SELECT Createtime,title,readcount,id,belong from gq_preferential WHERE topic= "Top" and id= "$id"), you will get an error.

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