The meaning of a variable that starts with an underscore in PHP

Named rules
Add one for private
Add two is usually the system default, the system is predefined, that is, the so-called:
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"Magic Method" and "Magic constant"
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★php constants that start and end with double underlines are "Magic constants":

The current line number in the __line__ file.

The full path and file name of the __file__ file.

The directory where the __dir__ file resides. If used in the included file, returns the directory where the included files are located. It is equivalent to DirName (__file__). Unless it is a root directory, the name in the directory does not include the trailing slash

Note: The above is from the "PHP Chinese Manual, language Reference, constant-and magic constant".


From the later version of PHP5, the PHP class will be able to use the Magic method.


PHP rules to start with two underscore (__) methods are preserved as a magic method, it is recommended that the function name is best not to start without __, unless it is to reload the existing magic method.

The Magic Methods in PHP are: __construct, __destruct, __call, __callstatic,__get, __set, __isset, __unset, __sleep, __wakeup, __toString, __set_state, __clone, __autoload

1, __get, __set

These two methods are designed for properties that are not declared in the class and their parent class.

__get ($property) This method is triggered when an undefined property is called, and the passed parameter is the property name that is accessed

__set ($property, $value) when assigning a value to an undefined property, this method is triggered, and the passed parameter is the property name and value that is set

There is no declaration here that the access control is a Proteced,private property (that is, a property without permission access) when using object invocation.

2, __isset, __unset

__isset ($property) This method is called when the Isset () function is called on an undefined property

__unset ($property) This method is called when the unset () function is called on an undefined property

As with the __get method and the __set method, there is no declaration here that the access control is a property of Proteced,private when using an object invocation (that is, a property without permission access)

3, __call

__call ($method, $arg _array) when calling an undefined method is to call this method

The undefined method here includes a method that does not have access, if the method does not exist in the parent class to find this method, if it does not exist in the parent class to call the __call () party of this class, if there is no __call () method in this class to find the __call () method in the parent class

4, __autoload

The __autoload function, which is called automatically when trying to use a class that has not yet been defined. By calling this function, the scripting engine has the last chance to load the required classes before PHP fails.

If you are defining a global auto-load class, you must register the processing class with the PHP standard library using the Spl_autoload_register () method:

<?PHPclassLoader {Static functionAutoload_class ($class _name)    {    //look for the right $class_name class, and introduce, no then throw an exception}    }        /** * Set automatic loading of objects * Spl_autoload_register-register given function as __autoload () implementation*/Spl_autoload_register (Array(' Loader ', ' Autoload_class ')); $a=NewTest ();//test does not use require to instantiate, to achieve automatic loading, many frameworks use this method to automatically load class?>

Note: Exceptions thrown in the __autoload function cannot be caught by a catch statement block and cause a fatal error, so the function itself should be captured.

5, __construct, __destruct

__construct constructs a method that calls this method when an object is created, and the advantage of using this method with respect to PHP4 is that the constructor can have a unique name, regardless of the name of the class in which it resides. So you don't need to change the name of the constructor when you change the name of the class.

__destruct destructor, PHP calls this method before the object is destroyed (that is, before it is purged from memory). By default, PHP simply frees the memory occupied by the object's properties and destroys the object-related resources, and the destructor allows you to execute arbitrary code to clear the memory after using an object. When PHP decides that your script is no longer related to the object, the destructor is called.

Within the namespace of a function, this occurs at the time of the function return.

For global variables, this happens at the end of the script.

If you want to explicitly destroy an object, you can assign any other value to the variable that points to the object. You typically assign a variable to be on duty to null or call unset.

6, __clone

The object assignment in PHP5 is to use a reference assignment, and if you want to copy an object then you need to use the Clone method, which is called by the object to call the __clone Magic method automatically, if the object replication requires some initialization to be performed, it can be implemented in the __clone method.

7, __tostring

The __tostring method is called automatically when an object is converted to a string, such as when using Echo to print an object.

If the class does not implement this method, the object cannot be printed through ECHO, otherwise it will be displayed: Catchable fatal Error:object of class test could not being converted to string in
This method must return a string.

Before PHP 5.2.0, the __tostring method will only take effect if you use Echo () or print () together. After PHP 5.2.0, you can take effect in any string environment (for example, with the%s modifier through printf (), but not in a non-string environment (such as using the%d modifier). From PHP 5.2.0, a e_recoverable_error error is reported if an object with an undefined __tostring method is converted to a string.

8, __sleep, __wakeup

__sleep serialization is used when

__wakeup Crossdress is called when the line is being serialized

Serialize () checks the class for a function that has the magic name __sleep. If so, the function will run before any serialization. It clears the object and should return an array containing all the variable names that should be serialized in the object.

The purpose of using __sleep is to close any database connections that an object might have, submit data for waiting, or perform similar cleanup tasks. In addition, this function is useful if you have very large objects and do not need to be fully stored.

Conversely, unserialize () checks for the existence of a function with the magic name __wakeup. If present, this function can reconstruct any resources that an object might have.

The purpose of using __wakeup is to reconstruct any database connections that may be lost in serialization and to handle other reinitialization tasks.

9, __set_state

This static method is called when Var_export () is called (Valid from PHP 5.1.0).

The only parameter to this method is an array that contains the by array (' property ' = = value, ...) The class properties of the format arrangement.

10, __invoke

The __invoke method is called automatically when an attempt is made to invoke an object in a way that invokes a function.

PHP5.3.0 above version Valid

11, __callstatic

It works like the __call () Magic Method, __callstatic () is to handle static method calls,

PHP5.3.0 above version Valid
PHP does strengthen the definition of the __callstatic () method; it must be public and must be declared static. Similarly, the __call () Magic method must be defined as public, and all other magic methods must do so.

The meaning of a variable that starts with an underscore in PHP