php中函数 isset(), empty(), is_null() 的区别[通俗易懂]

$myvariable; //此处你想定义一个变量，但未赋值。会有Notice: Undefined variable
echo $myvariable + 3; //使用这个变量出现：Notice: Undefined variable: myvariable in

$myvariable = NULL; 
echo $myvariable + 3; //这样就不会有问题了

$myvar = NULL; 
var_dump(is_null($myvar)); // TRUE
$myvar1; 
var_dump(is_null($myvar1)); // TRUE Notice: Undefined variable
$num = 520;
unset($num);
var_dump(is_null($num)); //TRUE Notice: Undefined variable
var_dump(is_null($some_undefined_var)); //TRUE Notice: Undefined variable

$myvar = 0; is_null($myvar); // FALSE
$myvar = FALSE; is_null($myvar); // FALSE
$myvar = ''; is_null($myvar); // FALSE

$myvar = NULL; isset($myvar); // FALSE 当一个变量被赋值为NULL时，就表示这个变量没有值
$myvar = 0; isset($myvar); // TRUE
$myvar = FALSE; isset($myvar); // TRUE
$myvar = ''; isset($myvar); // TRUE
isset($some_undefined_var); // FALSE Undefined variable

defined()：
bool defined ( string $name )

empty():
bool empty ( mixed $var )

$myvar = NULL; empty($myvar); // TRUE
$myvar = 0; empty($myvar); // TRUE
$myvar = FALSE; empty($myvar); // TRUE
$myvar = ''; empty($myvar); // TRUE
empty($some_undefined_var); // TRUE

if (isset($_GET['reply_id'] && $_GET['reply_user'])) {

报错:Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in line 42
改为这样:

 if ( isset($_GET['reply_id'], $_GET['reply_user']) ) {
      // code here
 }