How to use the php isset() function

The isset() function in php is used to detect whether the variable exists and is not NULL, the syntax is "isset (variable)"; if the variable exists and is not NULL, it returns TRUE, otherwise it returns FALSE. If a variable has been released using unset(), then judging by isset() will return FALSE.

The operating environment of this tutorial: windows7 system, PHP7.1 version, DELL G3 computer

PHP isset() function

isset()The function is used to detect whether the variable has been set and is not NULL.

Syntax

bool isset ( mixed $var [, mixed $... ] )

Parameter description:

• $var: the variable to be detected.

If multiple parameters are passed in at one time, isset() will only return TRUE when all parameters are set. The calculation process is from left to right. If an unset variable is encountered in the middle, it will be immediately stop.

Return value

If the variable does not exist, return FALSE

If the variable exists and its value is NULL, it also returns FALSE

If the variable exists and its value If it is not NULL, it will return TRUE

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

If it has been released using unset() After a variable is set(), 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.

Note: The null character ("\0") is not equivalent to PHP's NULL constant.

Warning: isset() can only be used with variables, because passing any other parameters will cause a parsing error. If you want to check whether a constant has been set, you can use the defined() function.

Example

<?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

// &#39;hello&#39; 等于 NULL，所以被认为是未赋值的。
// 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

?>

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