function find_sum_of_first_n_odd_numbers($n) {
  $sum = 0;
  for( $i = 1; $i <= 2*$n; $i+= 2) {
    $sum += $i;
  }
  return $sum;
}


echo find_sum_of_first_n_odd_numbers( 4 );

pow($n,2)  // pow() is the old-school call

$n**2  // is new-school

$number_of_odd_numbers=range(1,10);  // test cases

foreach($number_of_odd_numbers as $n){  // iterate test cases
    echo "$n => " , pow($n,2) , "n";
    //    ^^-input  ^^^^^^^^^--- square $n
}

1 => 1     // 1
2 => 4     // 1,3
3 => 9     // 1,3,5
4 => 16    // 1,3,5,7
5 => 25    // 1,3,5,7,9
6 => 36    // 1,3,5,7,9,11
7 => 49    // 1,3,5,7,9,11,13
8 => 64    // 1,3,5,7,9,11,13,15
9 => 81    // 1,3,5,7,9,11,13,15,17
10 => 100  // 1,3,5,7,9,11,13,15,17,19