PHPempty()isset()is_null()区别与性能比较_PHP教程

“” (an empty string)

0 (0 as an integer)

0.0 (0 as a float)

“0″ (0 as a string)

NULL

FALSE

array() (an empty array)

var $var; (a variable declared, but without a value in a class)

$a;
$b = false;
$c = '';
$d = 0;
$e = null;
$f = array();

?>

empty()

首先是empty的var_dump输出：

:
var_dump(empty($a));
var_dump(empty($b));
var_dump(empty($c));
var_dump(empty($d));
var_dump(empty($e));
var_dump(empty($f));
?>

程序输出为：
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)

从代码中可以看出，只要数据类型是否为空或假，empty()就输出true。
isset()

再看看isset的输出:
var_dump(isset($a));
var_dump(isset($b));
var_dump(isset($c));
var_dump(isset($d));
var_dump(isset($e));
var_dump(isset($f));

// 输出
bool(false)
bool(true)
bool(true)
bool(true)
bool(false)
bool(true)

可以看出isset()只能用来判断是否为NULL和未定义。
is_null()

最后是is_null的输出:
var_dump(is_null($a));
var_dump(is_null($b));
var_dump(is_null($c));
var_dump(is_null($d));
var_dump(is_null($e));
var_dump(is_null($f));

// 输出
bool(true)
bool(false)
bool(false)
bool(false)
bool(true)
bool(false)

$a=100;

$b="";

$c=null;

//isset检查

echo "isset","$a=$a",isset($a)?"define":"undefine","rn";

echo "isset","$b=$b",isset($b)?"define":"undefine","rn";

echo "isset","$c=$c",isset($c)?"define":"undefine","rn";

unset($b);

echo "isset","$b",isset($b)?"define":"undefine","rn";

$b=0;

echo "rnrn";

//empty检查

echo "empty","$a=$a",!empty($a)?"no empty":"empty","rn";

echo "empty","$b=$b",!empty($b)?"no empty":"empty","rn";

echo "empty","$c=$c",!empty($c)?"no empty":"empty","rn";

unset($b);

echo "empty","$b",!empty($b)?"no empty":"empty","rn";

$b=0;

echo "rnrn";

//is_null检查

echo "is_null","$a=$a",!is_null($a)?"no null":"null","rn";

echo "is_null","$b=$b",!is_null($b)?"no null":"null","rn";

echo "is_null","$c=$c",!is_null($c)?"no null":"null","rn";

unset($b);

echo "is_null","$b",is_null($b)?"no null":"null","rn";