PHP isset function

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isset

(PHP 3, PHP 4, PHP 5)

Returns TRUE if var exists; FALSE otherwise.

If a variable has been unset with unset(), it will no
longer be set. isset() will
return FALSE if testing a variable that has been set to NULL. Also
note that a NULL byte (“\0″) is not equivalent to
the PHP NULL constant.

Warning:
isset() only works with variables as passing anything
else will result in a parse error. For checking if
constants are set use the
defined() function.

 
$var = '';
 
// This will evaluate to TRUE so the text will be printed.
if (isset($var)) {
    echo "This var is set so I will print.";
}
 
// In the next examples we'll use var_dump to output
// the return value of isset().
 
$a = "test";
$b = "anothertest";
 
var_dump(isset($a));      // TRUE
var_dump(isset($a, $b)); // TRUE
 
unset ($a);
 
var_dump(isset($a));     // FALSE
var_dump(isset($a, $b)); // FALSE
 
$foo = NULL;
var_dump(isset($foo));   // FALSE

This also work for elements in arrays:

 
$a = array ('test' => 1, 'hello' => NULL);
 
var_dump(isset($a['test']));            // TRUE
var_dump(isset($a['foo']));             // FALSE
var_dump(isset($a['hello']));           // FALSE
 
// The key 'hello' equals NULL so is considered unset
// If you want to check for NULL key values then try: 
var_dump(array_key_exists('hello', $a)); // TRUE

Note: Because this is a
language construct and not a function, it cannot be called using
variable functions

See also empty(),
unset(),
defined(),
the type comparison tables,
array_key_exists(),
is_null(),
and the error control
@ operator.