This piece of code describes my problem:

<?

$variable = 0;

if ($variable == "some_string")
{
  print "This must be a bug?! Variable set as $variable.\n\n<br>";
  $variable++;
  if ($variable != "some_string")
  {
    print "But with variable set as $variable it works fine.\n\n";
  }
}
else
{
  print "This works as I expected.\n\n";
}

?>