public function testSomething (){
    ob_start();
    $class = new MyClass();
    $class->method();
    ob_clean();
}

<?php
public function testSomething (){
    ob_start();
    ob_implicit_flush(false); // turn off implicit flush

// Make your output below
    $class = new MyClass();
    $class->method();
// End of output

// store output into variable:
    $output = ob_get_contents();
}
?>

ob_implicit_flush(false);
ob_start();
/*
  ...
  ... do something that pushes countent to the output buffer
  ...
*/
$rendered = ob_get_contents();
ob_end_clean(); // Needed to clear the buffer