Example code for usage of the PHP isset () function and the unset () function

This article is mainly for PHP isset () and unset () functions of the use of a detailed introduction, the need for friends can come to the reference, I hope to be helpful to everyone

Isset
(PHP 3, PHP 4, PHP 5)

Isset--Detect if the variable is set

Describe
BOOL Isset (mixed var [, mixed Var [, ...])
Returns TRUE if Var exists, otherwise FALSE is returned.

If a variable has been freed with unset (), it will no longer be isset (). If you use Isset () to test a variable that is set to NULL, FALSE is returned. Also note that a null byte ("s") is not equivalent to a null constant in PHP.

Warning: isset () can only be used for variables, because passing any other parameter will result in parsing errors. To detect if a constant is set, use the defined () function.

The code is as follows:

<?php$var = ';//The result is TRUE, so the text behind will be printed. if (Isset ($var)) {     print "This var was set set so I'll print.";} In the example below, we will use Var_dump to output the return value of Isset (). $a = "Test"; $b = "Anothertest"; Var_dump (Isset ($a));      Truevar_dump (Isset ($a, $b)); Trueunset ($a); Var_dump (Isset ($a));     Falsevar_dump (Isset ($a, $b)); False$foo = Null;var_dump (Isset ($foo));   False?>

This is equally valid for the elements in the array:

The code is as follows:

<?php$a = Array (' Test ' = 1, ' hello ' = NULL); Var_dump (isset ($a [' Test ']);            Truevar_dump (isset ($a [' foo ']);             Falsevar_dump (isset ($a [' hello ']);           The value of the false//key ' hello ' is equal to NULL, so it is considered to be an unassigned value. If you want to detect NULL key values, you can try the methods below. Var_dump (array_key_exists (' hello ', $a)); True?>

Note: Because this is a language structure and not a function, it cannot be called by a "variable function".

Reasonable application of PHP function Isset () can help us to detect whether a variable is set. Returns FALSE if the variable does not exist, and returns TURE if the variable exists and the value is not NULL.

By learning the language of PHP, you should know that it is a function-based HTML scripting language. The large library of functions supports the implementation of PHP language functions. Here we introduce the relevant usage of PHP function isset ().

Format: bool Isset (mixed var [, mixed Var [, ...])

Function: Detect whether the variable is set

return value:

Returns FALSE if the variable does not exist
Returns FALSE if the variable exists and its value is null
Returns TURE if the variable exists and the value is not NULL

Returns TRUE if each item meets the previous requirement when checking multiple variables at the same time, otherwise the result is FALSE

Version: PHP 3, PHP 4, PHP 5

More information:

After you use Unset () to release a variable, it will no longer be isset ().
PHP functions Isset () can only be used for variables, and passing any other parameter will result in parsing errors.
Detects if a constant is set to use the defined () function.

Unset ()

Destroys the specified variable. Note in PHP 3, unset () returns True (actually an integer value of 1), whereas in PHP 4, unset () is no longer a real function: it is now a statement. Thus there is no return value, and attempting to get the return value of unset () will result in a parse error.