if（！$ variable）和if（isset（$ variable））之间有什么区别？

Question

if(!$variable)和if(isset($variable))之间if(!$variable)什么区别？

Answer 1

Well, the answer is pretty simple. isset($var) returns whether or not a variable exists and is not null, where !$var tells you if that variable is true , or anything that evaluates to true (such as a non-empty string). This is summarized in the first table of this documentation page .

Also, using !$var will output a notice that you're using an undefined variable, whereas isset($var) won't do that.

Mind you, they are two different things:

<?php
var_dump( isset($foo) ); // false.
var_dump( !$foo );       // true, but with a warning.

$foo = false;
var_dump( isset($foo) ); // true
var_dump( !$foo );       // true.

Answer 2

如果未设置变量，则在使用if（！Variable）时将收到警告。

Answer 3

They are two different statements

1. in the first you check if a variable is false hence the '!'
2. here you check if a variable is actually set to some value other then null.