How to find the list prime number from given number?

function prima($n){

  for($i=1;$i<=$n;$i++){  //numbers to be checked as prime

          $counter = 0; 
          for($j=1;$j<=$i;$j++){ //all divisible factors


                if($i % $j==0){ 

                      $counter++;
                }
          }

        //prime requires 2 rules ( divisible by 1 and divisible by itself)
        if($counter==2){

               print $i." is Prime <br/>";
        }
    }
} 

prima(20);

OUTPUT 

 2 is Prime 
 3 is Prime 
 5 is Prime 
 7 is Prime 
 11 is Prime 
 13 is Prime 
 17 is Prime 
 19 is Prime