<?php
include ('form1.php');
include ('form2.php');

class Main {
  function f1 () {
    echo('тест');
  }
}

$object = new Main;
if (isset($_GET['f1']))
  form1::f_form1();
elseif (isset($_GET['f2']))
  form2::f_form2();

?>

<?php
class form1 {
  function f_form1() {
    main::f1();
  }
}

$object = new form1;
if (isset($_GET['form1']))
  $object->f_form1();
?>

<?php

class form2 {
  function f_form2() {
      echo '<iframe name="I1" src="form1.php?form1=1">';
      echo '</iframe>';
  }
}
?>

<?php
include ('form1.php');
include ('form2.php');

class Main {
  function f_service () {
    echo('Многократно используемая сервисная функция<br>');
  }


  function f1 () {
    echo('Функция. запускаемая со страницы form1.php<br>');
    form1::f_form1();
  }
}

$object = new Main;

if (isset($_POST['f1']))
  $object->f1();
elseif (isset($_GET['f1']))
  form1::f_form1();
elseif (isset($_GET['f2']))
  form2::f_form2();

?>

<?php
class form1 {
  function f_form1() {
    echo '<form action="main.php" method="post">';   
      echo '<input name="f1" type="hidden" value=True>'; 
      main::f_service();
      echo '<input type=submit value="f1">';
    echo '</form>';
  }
}

$object = new form1;
if (isset($_GET['form1']))
  $object->f_form1();
?>

<?php

class form2 {
  function f_form2() {
      echo '<iframe name="I1" src="form1.php?form1=1">';
      echo '</iframe>';
  }
}
?>