Is there any reason to use isset()?

问题

Why should I use if (isset($var)) {} rather than just if ($var) {}? It seems to do the same thing and just take extra processing. Thanks!

回答1:

Reason

The reason is, isset() will return boolean and doesn't throw a warning when you check for the existence of variable and proceed. Also, there is a possibility that, a variable may have zero values:

1. false
2. 0
3. ""

But they will be already set.

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Example

$varb = false;
$vari = 0;
$vars = "";

isset($varb) // true
isset($vari) // true
isset($vars) // true

if ($varb) // false
if ($vari) // false
if ($vars) // false

回答2:

You use isset() to check if a variable has been declared.

The other method checks what value $var has. So if $var happened to contain false then the condition would be false but you wouldn't whether the variable wasn't set or the variable contained false.

来源：https://stackoverflow.com/questions/15286321/is-there-any-reason-to-use-isset