isset() / empty() / is_null() Since: PHP 4（2000）

// Checks that a variable is set and is not null.
isset($var, $var2, ...);

// Checks whether a variable is empty.
empty($var);

// Checks whether a variable is null.
is_null($var);

// Destroys a variable.
unset($var, $var2, ...);

<?php
// Use isset() to check whether a variable exists.
$name = "John Doe";
var_dump(isset($name)); // Outputs bool(true).
var_dump(isset($undefined)); // Outputs bool(false) because the variable is not defined.

// A null variable returns false from isset().
$value = null;
var_dump(isset($value)); // Outputs bool(false).

// You can check multiple variables at once.
$a = 1;
$b = 2;
var_dump(isset($a, $b)); // Outputs bool(true) because all variables are set.

// Use empty() to check whether a variable is empty.
var_dump(empty("")); // Outputs bool(true).
var_dump(empty("0")); // The string "0" is also considered empty.
var_dump(empty("abc")); // Outputs bool(false).

// A common pattern for validating form input.
$_POST['email'] = "user@example.com";
if (!empty($_POST['email'])) {
	echo "An email address has been entered."; // This line is output.
}

// Also useful for checking whether an array key exists.
$config = ['debug' => false, 'version' => '1.0'];
var_dump(isset($config['debug'])); // Outputs bool(true).
var_dump(isset($config['author'])); // Outputs bool(false) because the key does not exist.

// Use unset() to destroy a variable.
$temp = "temporary data";
unset($temp);
var_dump(isset($temp)); // Outputs bool(false) because the variable has been destroyed.

// is_null() checks for null only.
$val = null;
var_dump(is_null($val)); // Outputs bool(true).
var_dump(is_null("")); // Outputs bool(false) because an empty string is not null.

isset() checks that a variable is set and not null, while empty() checks whether a variable is empty. Because empty() treats "0" and 0 as empty, using it to validate numeric input can produce unexpected results. To check whether a value is a valid number, use is_numeric().

isset() and empty() are language constructs, so using them on undefined variables does not trigger a warning. In contrast, is_null() is a regular function, and using it on an undefined variable will produce a Warning.

A common pattern for validating form input is to use isset($_POST['key']) to confirm the key exists, and !empty($_POST['key']) to confirm a value was entered. For more on reading request data, see $_GET / $_POST.