~ $ php --version
PHP 5.3.8 (cli) (built: Dec  5 2011 21:24:09) 
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies

<?
  print "PHP Version is ".phpversion()."\n";

  print '+---------------';
  print '+-----------' ;
  print '+-----------' ;
  print "+\n";

  print '| value         ';
  print '| isset()   ' ;
  print '| empty()   ' ;
  print "|\n";

  print '+---------------';
  print '+-----------' ;
  print '+-----------' ;
  print "+\n";

  print '| $foobar       ';
  $foobar = NULL;
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '| $foobar = 0   ';
  $foobar = 0;
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '| $foobar = ""  ';
  $foobar = "";
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '| $foobar = 42  ';
  $foobar = 42;
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '| $foobar = "42"';
  $foobar = "42";
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '| $foobar = "x" ';
  $foobar = "x";
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '| $foobar = NULL';
  $foobar = NULL;
  if(isset($foobar)) print '| set       ' ;
  else               print '| not set   ' ;
  if(empty($foobar)) print '| empty     ' ;
  else               print '| not empty ' ;
  print "|\n";

  print '+---------------';
  print '+-----------' ;
  print '+-----------' ;
  print "+\n";

gives

PHP Version is 5.3.8
+---------------+-----------+-----------+
| value         | isset()   | empty()   |
+---------------+-----------+-----------+
| $foobar       | not set   | empty     |
| $foobar = 0   | set       | empty     |
| $foobar = ""  | set       | empty     |
| $foobar = 42  | set       | not empty |
| $foobar = "42"| set       | not empty |
| $foobar = "x" | set       | not empty |
| $foobar = NULL| not set   | empty     |
+---------------+-----------+-----------+