回调可以通过 callable 类型声明来表示。
一些函数如 call_user_func() 或 usort() 可以接受用户自定义的回调函数作为参数。回调函数不止可以是简单函数，还可以是对象的方法，包括静态类方法。
PHP是将函数以string形式传递的。 可以使用任何内置或用户自定义函数，但除了语言结构例如：array()，echo，empty()，eval()，exit()，isset()，list()，print 或 unset()。
一个已实例化的 object 的方法被作为 array 传递，下标 0 包含该 object，下标 1 包含方法名。 在同一个类里可以访问 protected 和 private 方法。
静态类方法可以不实例化 object 传递，只需要在下标为 0 的位置传递类名而不是
object ，或者传递 'ClassName::methodName'。
回调参数不仅可以使用普通的用户自定义函数，也接受 匿名函数 和 箭头函数。
注意:
从 PHP 8.1.0 开始，还可以使用 First-class 可调用语法 创建匿名函数。
通常情况下，任何实现了 __invoke() 的对象都可以传入回调参数。
示例 #1 回调函数示例
<?php
// 回调函数示范
function my_callback_function() {
echo 'hello world!';
}
// 回调方法示范
class MyClass {
static function myCallbackMethod() {
echo 'Hello World!';
}
}
// 类型 1：简单的回调
call_user_func('my_callback_function');
// 类型 2：静态类方法回调
call_user_func(array('MyClass', 'myCallbackMethod'));
// 类型 3：对象方法回调
$obj = new MyClass();
call_user_func(array($obj, 'myCallbackMethod'));
// 类型 4：静态类方法回调
call_user_func('MyClass::myCallbackMethod');
// 类型 5：父级静态类回调
class A {
public static function who() {
echo "A\n";
}
}
class B extends A {
public static function who() {
echo "B\n";
}
}
call_user_func(array('B', 'parent::who')); // A，从 PHP 8.2.0 起弃用。
// 类型 6：实现 __invoke 的对象用于回调
class C {
public function __invoke($name) {
echo 'Hello ', $name, "\n";
}
}
$c = new C();
call_user_func($c, 'PHP!');
?>
示例 #2 使用 Closure 的示例
<?php
// 闭包
$double = function($a) {
return $a * 2;
};
// 这是数字范围
$numbers = range(1, 5);
// 这里使用闭包作为回调，
// 将范围内的每个元素数值翻倍
$new_numbers = array_map($double, $numbers);
print implode(' ', $new_numbers);
?>以上示例会输出：
2 4 6 8 10
注意:
在函数中注册有多个回调内容时(如使用 call_user_func() 与 call_user_func_array())，如在前一个回调中有未捕获的异常，其后的将不再被调用。
You can also use the $this variable to specify a callback:
<?php
class MyClass {
public $property = 'Hello World!';
public function MyMethod()
{
call_user_func(array($this, 'myCallbackMethod'));
}
public function MyCallbackMethod()
{
echo $this->property;
}
}
?>When specifying a call back in array notation (ie. array($this, "myfunc") ) the method can be private if called from inside the class, but if you call it from outside you'll get a warning:
<?php
class mc {
public function go(array $arr) {
array_walk($arr, array($this, "walkIt"));
}
private function walkIt($val) {
echo $val . "<br />";
}
public function export() {
return array($this, 'walkIt');
}
}
$data = array(1,2,3,4);
$m = new mc;
$m->go($data); // valid
array_walk($data, $m->export()); // will generate warning
?>
Output:
1<br />2<br />3<br />4<br />
Warning: array_walk() expects parameter 2 to be a valid callback, cannot access private method mc::walkIt() in /in/tfh7f on line 22Performance note: The callable type hint, like is_callable(), will trigger an autoload of the class if the value looks like a static method callback.A note on differences when calling callbacks as "variable functions" without the use of call_user_func() (e.g. "<?php $callback = 'printf'; $callback('Hello World!') ?>"):
- Using the name of a function as string has worked since at least 4.3.0
- Calling anonymous functions and invokable objects has worked since 5.3.0
- Using the array structure [$object, 'method'] has worked since 5.4.0
Note, however, that the following are not supported when calling callbacks as variable functions, even though they are supported by call_user_func():
- Calling static class methods via strings such as 'foo::doStuff'
- Calling parent method using the [$object, 'parent::method'] array structure
All of these cases are correctly recognized as callbacks by the 'callable' type hint, however. Thus, the following code will produce an error "Fatal error: Call to undefined function foo::doStuff() in /tmp/code.php on line 4":
<?php
class foo {
static function callIt(callable $callback) {
$callback();
}
static function doStuff() {
echo "Hello World!";
}
}
foo::callIt('foo::doStuff');
?>
The code would work fine, if we replaced the '$callback()' with 'call_user_func($callback)' or if we used the array ['foo', 'doStuff'] as the callback instead.You can use 'self::methodName' as a callable, but this is dangerous. Consider this example:
<?php
class Foo {
public static function doAwesomeThings() {
FunctionCaller::callIt('self::someAwesomeMethod');
}
public static function someAwesomeMethod() {
// fantastic code goes here.
}
}
class FunctionCaller {
public static function callIt(callable $func) {
call_user_func($func);
}
}
Foo::doAwesomeThings();
?>
This results in an error:
Warning: class 'FunctionCaller' does not have a method 'someAwesomeMethod'.
For this reason you should always use the full class name:
<?php
FunctionCaller::callIt('Foo::someAwesomeMethod');
?>
I believe this is because there is no way for FunctionCaller to know that the string 'self' at one point referred to to `Foo`.you can pass an object as a callable if its class defines the __invoke() magic method..I needed a function that would determine the type of callable being passed, and, eventually,
normalized it to some extent. Here's what I came up with:
<?php
/**
* The callable types and normalizations are given in the table below:
*
* Callable | Normalization | Type
* ---------------------------------+---------------------------------+--------------
* function (...) use (...) {...} | function (...) use (...) {...} | 'closure'
* $object | $object | 'invocable'
* "function" | "function" | 'function'
* "class::method" | ["class", "method"] | 'static'
* ["class", "parent::method"] | ["parent of class", "method"] | 'static'
* ["class", "self::method"] | ["class", "method"] | 'static'
* ["class", "method"] | ["class", "method"] | 'static'
* [$object, "parent::method"] | [$object, "parent::method"] | 'object'
* [$object, "self::method"] | [$object, "method"] | 'object'
* [$object, "method"] | [$object, "method"] | 'object'
* ---------------------------------+---------------------------------+--------------
* other callable | idem | 'unknown'
* ---------------------------------+---------------------------------+--------------
* not a callable | null | false
*
* If the "strict" parameter is set to true, additional checks are
* performed, in particular:
* - when a callable string of the form "class::method" or a callable array
* of the form ["class", "method"] is given, the method must be a static one,
* - when a callable array of the form [$object, "method"] is given, the
* method must be a non-static one.
*
*/
function callableType($callable, $strict = true, callable& $norm = null) {
if (!is_callable($callable)) {
switch (true) {
case is_object($callable):
$norm = $callable;
return 'Closure' === get_class($callable) ? 'closure' : 'invocable';
case is_string($callable):
$m = null;
if (preg_match('~^(?<class>[a-z_][a-z0-9_]*)::(?<method>[a-z_][a-z0-9_]*)$~i', $callable, $m)) {
list($left, $right) = [$m['class'], $m['method']];
if (!$strict || (new \ReflectionMethod($left, $right))->isStatic()) {
$norm = [$left, $right];
return 'static';
}
} else {
$norm = $callable;
return 'function';
}
break;
case is_array($callable):
$m = null;
if (preg_match('~^(:?(?<reference>self|parent)::)?(?<method>[a-z_][a-z0-9_]*)$~i', $callable[1], $m)) {
if (is_string($callable[0])) {
if ('parent' === strtolower($m['reference'])) {
list($left, $right) = [get_parent_class($callable[0]), $m['method']];
} else {
list($left, $right) = [$callable[0], $m['method']];
}
if (!$strict || (new \ReflectionMethod($left, $right))->isStatic()) {
$norm = [$left, $right];
return 'static';
}
} else {
if ('self' === strtolower($m['reference'])) {
list($left, $right) = [$callable[0], $m['method']];
} else {
list($left, $right) = $callable;
}
if (!$strict || !(new \ReflectionMethod($left, $right))->isStatic()) {
$norm = [$left, $right];
return 'object';
}
}
}
break;
}
$norm = $callable;
return 'unknown';
}
$norm = null;
return false;
}
?>
Hope someone else finds it useful.If you pass a callable method to a function with a callable type declaration, the error message is misleading:
<?php
class X {
protected function foo(): void {}
}
function bar(callable $c) {}
$x = new X;
$c = [$x, 'foo'];
bar($c);
?>
Error message will be something like "Argument #1 ($c) must be of type callable, array given" while the actual problem here is only the visibility of method "foo". All you need to do is changing it to public (or use a different approach, e.g. with a Closure).When trying to make a callable from a function name located in a namespace, you MUST give the fully qualified function name (regardless of the current namespace or use statements).
<?php
namespace MyNamespace;
function doSomethingFancy($arg1)
{
// do something...
}
$values = [1, 2, 3];
array_map('doSomethingFancy', $values);
// array_map() expects parameter 1 to be a valid callback, function 'doSomethingFancy' not found or invalid function name
array_map('MyNamespace\doSomethingFancy', $values);
// => [..., ..., ...]I tried many possible ways of calling functions by function name directly and assigned to a variable on 3v4l. Not mentioned yet, it is possible to use an array as a caller, at least since PHP 7.1.25. The following script contains all the information I gained:
<?php
// Call function via function name:
// Basics:
// A function can also be called by using its string name:
function callbackFunc() {
echo 'Hello World';
}
'callbackFunc'(); // Hello World
// A function can also be called if its name is assigned to a variable:
function callbackFunc() {
echo 'Hello World';
}
$funcName = 'callbackFunc';
$funcName(); // Hello World
// Static class method:
// It is also possible to call a public static class method via 'ClassName::functioName' notation:
class A {
public static function callbackMethod() {
echo "Hello World\n";
}
}
'A::callbackMethod'(); // Hello World
$funcName = 'A::callbackMethod';
$funcName(); // Hello World
// Non static class method:
// It is also possible to call non static class methods by creating an array which first element is the object the method should be called on and the second element is the non static method to be called. The array can directly be used as a caller:
class A {
private $prop = "Hello World\n";
public function callbackMethod() {
echo $this->prop;
}
}
$a = new A;
[$a, 'callbackMethod']();
$funcCallArr = [$a, 'callbackMethod'];
$funcCallArr();
// Of course this also works inside the class with '$this':
class A {
private function privCallback() {
echo 'Private';
}
public function privCallbackCaller($funcName) {
[$this, $funcName]();
}
}
(new A)->privCallbackCaller('privCallback'); // Private
?>@edanschwartz at gmail dot com
You can use ::class property to always indicate the class you're in when using static methods:
<?php
class Foo {
public static function doAwesomeThings() {
FunctionCaller::callIt(self::class . '::someAwesomeMethod');
}
public static function someAwesomeMethod() {
// fantastic code goes here.
}
}
class FunctionCaller {
public static function callIt(callable $func) {
call_user_func($func);
}
}
Foo::doAwesomeThings();
?>Having read this line in the manual above,
"A method of an instantiated object is passed as an array containing an object at index 0 and the method name at index 1. Accessing protected and private methods from within a class is allowed."
I decided to do some testing to see if I could access private methods using the call_user_func methods. Thankfully not, but for completeness here is my test which also covers using static and object contexts
<?php
class foo {
public static $isInstance = false;
public function __construct() {
self::$isInstance = true;
}
public function bar() {
var_dump(self::$isInstance);
echo __METHOD__;
}
private function baz() {
var_dump(self::$isInstance);
echo __METHOD__;
}
public function qux() {
$this->baz();
}
public function quux() {
self::baz();
}
}
call_user_func(['foo','bar']); //fase, foo:bar
call_user_func(['foo','baz']); //warning, cannot access private method
call_user_func(['foo','quux']); //false, foo::baz
call_user_func(['foo','qux']); //fatal, Using $this when not in object context
$foo = new foo;
call_user_func([$foo,'bar']); //true, foo::bar
call_user_func([$foo,'baz']); //warning, cannot access private method
call_user_func([$foo,'qux']); //true, foo::baz
call_user_func(['foo','bar']); //true, foo::bar (static call, yet $isInstance is true)
?>