Invalid argument supplied for foreach() issue in laravel

Question

My query in controller page

$d= "DB::table('users')->where('gender',$gender)";    
if(!empty($age)){
$d.= "->where('age','>',$age)";
}    
$d.="->paginate(20)";

if age is not empty that time only want add age to the query. but query showing Invalid argument supplied for foreach() error. Without double quotes query running but unable to avoid empty age variable.

Answer 1

Shorter version

 $d = DB::table('users')->where('gender',$gender); 

 $d = empty($age) ? $d->paginate(20) : $d->where('age','>',$age)->paginate(20);

Answer 2

Please try it:

$d = DB::table('users')->where('gender',$gender);    
if(! empty($age)) {
    $d = $d->where('age','>',$age);
}    
$d = $d->paginate(20);