isset()和__isset()之间有什么区别？

Question

我需要了解魔法功能__isset()和正常功能isset().实际上php语言构造isset() 和php魔术方法的真正区别是什么__isset()？当我谷歌它,他们告诉这__isset()是一个神奇的功能.常见的php函数和php中的魔术函数有什么区别？

Answer 1

isset()

它是一种语言结构,用于检查变量或类属性的初始化:

$a = 10;

isset($a);     // true
isset($a, $b); // false

class Test
{
    public $prop = 10;
}

$obj = new Test;
isset($obj->prop); // true

__isset()

这是一个神奇的方法被调用时isset()或empty()检查不存在或无法访问类的属性:

class Test
{
    public function __isset($name) {
        echo "Non-existent property '$name'";
    }
}

$obj = new Test;
isset($obj->prop); // prints "Non-existent property 'prop'" and return false

区别:

           isset()                               __isset()

Language construct                    | Magic method
                                      |
Always return bool                    | Result depends on custom logic*
                                      |
Must be invoked in code               | Called automatically by event
                                      |
Unlimited number of parameters        | Has only one parameter
                                      |
Can be used in any scope              | Must be defined as method**
                                      |
Is a reserved keyword                 | Not a reserved keyword
                                      |
Can't be redefined (Parse error)      | Can be redefined in extended class***

__isset()无论如何,结果将被自动转换为bool.

实际上你可以定义自定义函数,__isset()但它与魔术方法无关.

看这个例子.

魔术方法

与通用函数不同,只能在类作用域中定义,并且在特定事件上自动调用,例如:不可访问的方法调用,类序列化,unset()用于不可访问的属性等等.另请参阅此官方文档:重载.

Answer 2

__isset是神奇的方法.魔术方法是内部调用的方法.

考虑以下代码

<?php
// Declare a simple class
class TestClass
{
    public $foo;

    public function __construct($foo)
    {
        $this->foo = $foo;
    }

    public function __toString()
    {
        return $this->foo;
    }
}

$class = new TestClass('Hello');
echo $class;
?>

这里_ toString是魔术方法,但你不会这称呼它.当行echo $ class; 被执行.PHP知道现在我应该将$ class对象视为字符串,并将任何对象视为字符串调用 _toString方法,如果在该类中实现的话.

所有神奇的方法都是以间接方式调用的.

另一个例子如下

<?php
class CallableClass
{
    public function __invoke($x)
    {
        var_dump($x);
    }
}
$obj = new CallableClass;
$obj(5);
var_dump(is_callable($obj));
?>

同样,在上面的代码中,var_dump(is_callable($ obj)); 间接调用__invoke魔法.

Answer 3

首先让我告诉你isset()函数的作用.isset()函数检查值是否已设置或是否为空._ isset()函数是PHP中的魔术方法.开头的任何带有"_ "的函数都是PHP中的魔术方法.现在,通过在不可访问的属性上调用isset()或empty()来调用__isset(),我指的是那些尚未在类中定义并在运行时显式定义的属性.这是一段代码,可以让你更好地理解它:

<?php
class PropertyTest
{
    /**  Location for overloaded data.  */
    private $data = array();

    /**  Overloading not used on declared properties.  */
    public $declared = 1;

    /**  Overloading only used on this when accessed outside the class.  */
    private $hidden = 2;

    public function __set($name, $value)
    {
        echo "Setting '$name' to '$value'\n";
        $this->data[$name] = $value;
    }

    public function __get($name)
    {
        echo "Getting '$name'\n";
        if (array_key_exists($name, $this->data)) {
            return $this->data[$name];
        }

        $trace = debug_backtrace();
        trigger_error(
            'Undefined property via __get(): ' . $name .
            ' in ' . $trace[0]['file'] .
            ' on line ' . $trace[0]['line'],
            E_USER_NOTICE);
        return null;
    }

    /**  As of PHP 5.1.0  */
    public function __isset($name)
    {
        echo "Is '$name' set?\n";
        return isset($this->data[$name]);
    }

    /**  As of PHP 5.1.0  */
    public function __unset($name)
    {
        echo "Unsetting '$name'\n";
        unset($this->data[$name]);
    }

    /**  Not a magic method, just here for example.  */
    public function getHidden()
    {
        return $this->hidden;
    }
}


echo "<pre>\n";

$obj = new PropertyTest;

$obj->a = 1;
echo $obj->a . "\n\n";

var_dump(isset($obj->a));
unset($obj->a);
var_dump(isset($obj->a));
echo "\n";

echo $obj->declared . "\n\n";

echo "Let's experiment with the private property named 'hidden':\n";
echo "Privates are visible inside the class, so __get() not used...\n";
echo $obj->getHidden() . "\n";
echo "Privates not visible outside of class, so __get() is used...\n";
echo $obj->hidden . "\n";
?>