What is the difference between isset() and __isset()?

Question

I need to know about magic function __isset() and normal function isset(). Actually what is the real difference between php language construct isset() and php magic method __isset() ? When I google it they told that __isset() is a magic function. What are difference between common php functions and magic functions in php?

Answer 1

isset()

It is a language construct that checks the initialization of variables or class properties:

$a = 10;  isset($a);     // true isset($a, $b); // false  class Test {     public $prop = 10; }  $obj = new Test; isset($obj->prop); // true 

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__isset()

It is a magic method that is invoked when isset() or empty() check non-existent or inaccessible class property:

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 

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Difference:

           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() result anyway will be automatically casted as bool.

Actually you can define custom function __isset() but it has nothing to do with the magic method.

See this example.

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Magic Methods

Unlike common functions can be defined only in class scope and invoked automatically on specific events such as: inaccessible method invocation, class serialization, when unset() used on inaccessible properties and so on. See also this official documentation: Overloading.

Answer 2

__isset is the magic method. Magic methods are the methods called internally.

Consider following code

<?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; ?> 

here _toString is magic method but you will not call that. When the line echo $class; is executed. PHP knows that now I should treat $class object as a string and to treat any object as the string call_toString method, if implemented in that class.

All magic methods called like this in indirect way.

Another example as follow

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

Similarly, in above code , var_dump(is_callable($obj)); invokes __invoke magic method indirectly.