Detailed explanation of php pseudo-types

php pseudo-types (pseudo-types) are used in PHP documents to indicate the types and values ​​that parameters can use. Please note that they are not native types in the PHP language. Therefore, pseudo-types cannot be used for type constraints (typehint) in custom functions.

mixed

mixed indicates that a parameter can accept multiple different (but not necessarily all) types.
For example, gettype() can accept all PHP types, str_replace() can accept strings and arrays.

number

number indicates that a parameter can be integer or float.

callback

The callback pseudo-type was used before the callable type was introduced in PHP 5.4. Both have exactly the same meaning.

Some functions such as call_user_func() or usort() can accept user-defined callback functions as parameters. The callback function can not only be a simple function, but also an object method, including static class methods.
For example, the call_user_func() function can receive a user-defined function as a parameter, which is a built-in function of PHP. The callback function can not only be a function, but also a method of an object and a method of a static class. A PHP function is passed as a function name string. Any built-in or user-defined function can be passed, except language structures such as array(), echo(), empty(), eval(), exit(), isset() ,list(),print(),unset(), etc.
Refer here for examples

//call_user_func()函数用法如下：function my_callback_function(){  
        echo "my is callback function.";  
    }  

    //回调方法  
    class MyClass{  
        static function myCallbackMethod(){  
            echo "my is callback method.";  
        }  
    }  

    //num1:回调函数  
    call_user_func('my_callback_function');  

    //num2:静态类的方法  
    call_user_func(array('MyClass','myCallbackMethod'));  

    //num3:对象方法  
    $obj = new MyClass();  
    call_user_func(array($obj,'myCallbackMethod'));  

    //num4:静态类的方法（5.3.0以上）  
    call_user_func('MyClass::myCallbackMethod');  

    //num5:相对静态方法调用  
    class A {  
        public static function who(){  
            echo "A";  
        }  
    }  

    class B extends A {  
        public static function who(){  
            echo "B";  
        }  
    }  

    call_user_func(array('B','parent::who'));   //输出B  

    //num6:调用时传入参数  
    function test($a,$b){  
        echo $a;  
        echo $b;  
    }  

    call_user_function('test','1','2'); //输出1 2  

    //num7:调用类方法传入参数  
    class a(){  
        function b($c){  
            echo $c;  
        }  
    }  

    call_user_func(array('a','b'),'1'); //输出1  

    //num8:call_user_func_array与call_user_func很相似，只不过换了一种方式传递参数，让参数结构更清晰：  
    function a($b,$c){  
        echo $b;  
        echo $c;  
    }  

    call_user_func_array('a',array('1','2'));   //输出1 2  

    //num9:用call_user_func_array调用类内部方法  
    class A(){  
        function a($b,$c){  
            echo $b;  
            echo $c;  
        }  
    }  

    call_user_func_array(array('A','a'),array('1','2'));    //输出1 2

array|object

array|object means that the parameter can be either array or object.

void

void as a return type means that the return value of the function is useless. void as a parameter list means that the function does not accept any parameters.

…

In the function prototype, $… means etc. Use this variable name when a function can accept any number of parameters.

<?phpfunction sum(...$numbers) {
    $acc = 0;    foreach ($numbers as $n) {        $acc += $n;
    }    return $acc;
}echo sum(1, 2, 3, 4);?>

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