Detailed explanation of how to use isset() and unset() functions in php

This article mainly introduces the usage of isset() and unset() functions in PHP in detail. Friends who need it can come and refer to it. I hope it will be helpful to everyone

isset
(PHP 3, PHP 4, PHP 5)

isset -- Check whether the variable is set

Description
bool isset (mixed var [, mixed var [, ...]])Returns TRUE if var exists, otherwise returns FALSE.

If a variable has been released using unset(), it will no longer be isset(). If you use isset() to test a variable that is set to NULL, it will return FALSE. Also note that a NULL byte ("\0") is not equivalent to PHP's NULL constant.

Warning: isset() can only be used with variables, because passing any

other arguments will cause a parsing error. If you want to detect whether a constant has been set, use the defined() function.

The code is as follows:

<?php
$var = &#39;&#39;;
// 结果为 TRUE，所以后边的文本将被打印出来。
if (isset($var)) {
     print "This var is set set so I will print.";
}
// 在后边的例子中，我们将使用 var_dump 输出 isset() 的返回值。
$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 is also valid for elements in the array:

The code is as follows:

<?php
$a = array (&#39;test&#39; => 1, &#39;hello&#39; => NULL);
var_dump( isset ($a[&#39;test&#39;]) );            // TRUE
var_dump( isset ($a[&#39;foo&#39;]) );             // FALSE
var_dump( isset ($a[&#39;hello&#39;]) );           // FALSE
// 键 &#39;hello&#39; 的值等于 NULL，所以被认为是未置值的。
// 如果想检测 NULL 键值，可以试试下边的方法。 
var_dump( array_key_exists(&#39;hello&#39;, $a) ); // TRUE
?>

Note: Since this is a language construct and not a function, it cannot be called from "variable functions".

Rational application

PHP functionisset() can help us detect whether the variable is set. Returns FALSE if the variable does not exist, and TRUE if the variable exists and is not NULL.

Through learning the PHP language, you should know that it is a

HTML script language based on functions. A huge function library supports the implementation of PHP language functions. Below we introduce to you the relevant usage of the PHP function isset().

Format: bool isset (mixed var [, mixed var [, ...]] )

Function: Check whether the variable is set

Return value:

If the variable does not exist, return FALSE

If the variable exists and its value is NULL, it will also return FALSE
If the variable exists and its value is not NULL, it will return TURE

Check at the same time When there are multiple variables, TRUE will be returned only when each single item meets the previous requirement, otherwise the result will be FALSE

Version: PHP 3, PHP 4, PHP 5

More instructions:

After using unset() to release a variable, it will no longer be isset().

PHP function isset() can only be used for variables. Passing any other parameters will cause a parsing error.
To detect whether a constant has been set, use the defined() function.

unset()

Destroy the specified variable. Note that in PHP 3, unset() will return TRUE (actually the integer value 1), while in PHP 4, unset() is no longer a real function: it is now a statement. There is no return value, and trying to get the return value of unset() will result in a parsing error.

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