if condition in laravel query

some time some variables are null so how check the variable is not null and apply direct in query

 $fiter_products = DB::table('products')->DISTINCT('modalid')->DISTINCT('brandid')->select('rimdiameter','modalid','modalname1','modalname2','image1','brand','minprice','maxprice')->where('hubbore','>=',$centre_bore)->where('boltpattern',$boltptn)->where('rimdiameter', $diameter)->where('rimwidth', $width)->where('rimwidthfront', $frontwid)->where('construction', $construct)->where('modalname2', $color)->where('brand', $brand)->get(); 

any way to solve this issue ?