isset（$ var）==“ Test”和isset（$ var）&& $ var =='Test“和有什么不一样

Question

isset($var) == "Test"和isset($var) && $var == 'Test"什么区别？

Answer 1

The first one makes no sense. As you can see isset returns a Boolean. So isset($var) == "Test" as I said evaluates a bool against a string.

With the evaluation isset($var) && $var == 'Test' PHP first checks if the variable $var is defined and then if that value is equal to the string 'test'.

Calling just $var == 'Test without ensuring that it is set will result in an 'Undefined variable' notice. If you are not sure and don't want a noisy log then you can check with isset.

Answer 2

Here a short example:

$var = "Chuck Test";

var_dump(isset($var)); // bool(true)
var_dump(isset($undefined)); // bool(false)

var_dump(isset($var) == "Chuck Test"); // bool(true)
var_dump(isset($var) && $var == "Chuck Test"); // bool(true)
var_dump(isset($undefined) == "Chuck Test"); // bool(false)
var_dump(isset($undefined) && $undefined == "Chuck Test"); // bool(false)

it looks like they are equivalent but they aren't:

var_dump(isset($var) == "Chuck Testa"); // bool(true) !!!
var_dump(isset($var) && $var == "Chuck Testa"); // bool(false)

because isset() returns true or false , and an non-empty string compared to true results in true .

So better use the isset($var) && $var == "Test" variant, because it does what you would expect.

Answer 3

The function isset() returns true (boolean) if a variable is set. Now when you compare a boolean == "Test", it is bogus. So to check whether your variable is set and has the value of 'Test' you should use isset($var) && $var == 'Test' . But I don't see why you should not do `$var == 'Test'. Does the interpreter complain about uninitialized variable that way?

Read the following, ask any question if you do not understand what the function isset actually does: http://php.net/manual/en/function.isset.php