PHP Check Prime Number

<?php
  /*define a function that checks if a given number is prime or not.
  * @number int
  * return boolean (true/false)
  */
  function is_prime($number)
  {
    if ($number == 1)
      return false;

    for ($i = 2; $i <= $number/2; $i++)
    {
      if ($number % $i == 0)
        return false;
    }

    return true;
  }

  $number = 33;

  /*now test*/
  if (is_prime($number))
    echo "$number is a prime number.";
  else
    echo "$number is not a prime number.";

  echo '<br>';
  /*another test*/
  $number = 47;
  if (is_prime($number))
    echo "$number is a prime number.";
  else
    echo "$number is not a prime number.";
?>

Output

33 is not a prime number.
47 is a prime number.