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Testing, setting, and unsetting variables

During the running of a PHP script, a variable may be in an unset state or may not yet be defined. PHP provides the isset( ) function and the empty( ) language construct to test the state of variables:

boolean isset(mixed var)
boolean empty(mixed var)

isset( ) tests if a variable has been set with a non-null value, while empty( ) tests if a variable has a value. The two are different, as shown by the following code:

$var = "test";

// prints: "Variable is Set"
if (isset($var)) echo "Variable is Set";

// does not print
if (empty($var)) echo "Variable is Empty";

A variable can be explicitly destroyed using unset( ):

unset(mixed var [, mixed var [, ...]])

After the call to unset in the following example, $var is no longer defined:

$var = "foo";

// Later in the script
unset($var);

// Does not print
if (isset($var)) echo "Variable is Set";

Another way to test that a variable is empty is to force it to the Boolean type using the (bool) cast operator discussed earlier. The example interprets the $var variable as type Boolean, which is equivalent to testing for !empty($var):

$var = "foo";

// Both lines are printed
if ((bool)$var)    echo "Variable is not Empty";
if (!empty($var))  echo "Variable is not Empty";

Table 2-2 show the return values for isset($var), empty($var), and (bool)$var when the variable $var is tested. Some of the results may be unexpected: when $var is set to "0", empty( ) returns true.

Table 2-2. Expression values

State of the variable $var	isset($var)	empty($var)	(bool)$var
$var = null;	false	true	false
$var = 0;	true	true	false
$var = true	true	false	true
$var = false	true	true	false
$var = "0";	true	true	false
$var = "";	true	true	false
$var = "foo";	true	false	true
$var = array( );	true	true	false
unset $var;	false	true	false

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