Example code to see how much memory a small class consumes.

class-memory-test.php

echo "<br>";
echo memory_get_usage();

class Foobar {
  var $a = 'abcdefghijklmnopqrstuvwxyz';
  var $b = 1000;
  var $c = false;
  var $d;
  function __construct() {
    $this->d = array(
      'a' => $this->a,
      'b' => $this->b,
      'c' => $this->c,
    );
  }

}
$fb= new Foobar();

echo "<br>";
echo memory_get_usage();

class Foobar1 {
}
echo "<br>";
echo memory_get_usage();