Let's test those comparisons! 

bar isn't set yet..

------------------------------------
foo= bar and bar= "" 
  !$bar is TRUE
  empty($bar) is TRUE
  $bar=="" is TRUE
  $bar==0 is TRUE
  $bar is a string! 
  ** It's an empty string! **
  It's still an empty string! **

------------------------------------
foo= bar and bar= 0 
  !$bar is TRUE
  empty($bar) is TRUE
  $bar=="" is TRUE
  $bar==0 is TRUE

------------------------------------
foo= bar and bar= "0" 
  !$bar is TRUE
  empty($bar) is TRUE
  $bar==0 is TRUE
  $bar is a string! 

------------------------------------
foo= bar and bar= 1 

--------------------- Source: ----

<?php
header("Content-type: text/plain");
print "Let's test those comparisons! \n\n";

ini_set("error_reporting", E_ALL);

function wringer($quux) {
	if ( !$quux ) {
		print "  !\$bar is TRUE\n";
		}

	if ( empty($quux) ) {
		print "  empty(\$bar) is TRUE\n ";
		}

	if ( $quux=="" ) {
		print " \$bar==\"\" is TRUE\n ";
		}

	if ( $quux==0) {
		print " \$bar==0 is TRUE\n";
		}

	if ( is_string($quux)) {
		print "  \$bar is a string! \n";
		}
		
	if ( is_string($quux) && $quux=='' ) {
		print "  ** It's an empty string! **\n";
		}
	
	if ( $quux==='' ) {
		print "  It's still an empty string! **\n";
		}
	}

$foo= "bar";
if ( isset($$foo) ) {
	if ( is_string($$foo) && $foo=='' ) print "it's an empty string!\n";
	}
else print "bar isn't set yet..\n";

print "\n------------------------------------\n";
$bar= "";
print "foo= $foo and bar= \"\" \n";
wringer($$foo);

print "\n------------------------------------\n";
$bar= 0;
print "foo= $foo and bar= $bar \n";
wringer($$foo);

print "\n------------------------------------\n";
$bar= "0";
print "foo= $foo and bar= \"0\" \n";
wringer($$foo);

print "\n------------------------------------\n";
$bar= 1;
print "foo= $foo and bar= $bar \n";
wringer($$foo);

print "\n--------------------- Source: ----\n\n";
print file_get_contents($_SERVER['SCRIPT_FILENAME']);