Insert variable in mysqli update query error

Question

I am trying to insert a variable in mysqli update query, if I add direct number in update query it's working but if I add the number into a variable and insert the variable into the statement it's not working.

This query is working:

$query = 'update `'.$cars.'` set sale_time="'.$time.'" where model=0010';

But this isn't working:

$query = 'update `'.$cars.'` set sale_time="'.$time.'" where model="'.$model.'"';