Documentation states
void ob_start ( [callback output_callback])

In reality since 4.3.2 this function returns a boolean value. It returns true if output buffering started successfully, and false if some problem occured. This problem can be output callback function that does not exist. Earlier versions of PHP started output buffering with default handler in this case.

Code to show backward incompatibility:
<?
  ob_start();
  echo "AAA";
  $a = ob_get_contents();
  $var = ob_start("thereisnosuchfunction");
  echo "BBB";
  $b = ob_get_contents();
  ob_end_clean();
  ob_end_clean();
  echo $b.'---'.$a;
  var_dump($var);
?>

4.3.2 outputs
AAABBB---AAAbool(false)
earlier versions output
BBB---AAAbool(true)