PHP 手册
后退章 17. 函数前进


变量函数

PHP 支持变量函数的概念。这意味着如果一个变量名后有圆括号，PHP 将寻找与变量的值同名的函数，并且将尝试执行它。除了别的事情以外，这个可以被用于实现回调函数，函数表等等。

变量函数不能用于语言结构，例如 echo()，print()，unset()，isset()，empty()，include()，require() 以及类似的语句。需要使用自己的外壳函数来将这些结构用作变量函数。

例子 17-14. 变量函数示例

<?php
function foo() {
    echo "In foo()<br />\n";
}

function bar($arg = '') {
    echo "In bar(); argument was '$arg'.<br />\n";
}

// This is a wrapper function around echo
function echoit($string)
{
    echo $string;
}

$func = 'foo';
$func();        // This calls foo()

$func = 'bar';
$func('test');  // This calls bar()

$func = 'echoit';
$func('test');  // This calls echoit()
?>

还可以利用变量函数的特性来调用一个对象的方法。

例子 17-15. 变量方法范例

<?php
class Foo
{
    function Variable()
    {
        $name = 'Bar';
        $this->$name(); // This calls the Bar() method
    }

    function Bar()
    {
        echo "This is Bar";
    }
}

$foo = new Foo();
$funcname = "Variable";
$foo->$funcname();   // This calls $foo->Variable()

?>

请参阅 call_user_func()，可变变量和 function_exists()。


add a note add a note User Contributed Notes
boards at gmail dot com
23-Jan-2006 02:07
If you want to call a static function (PHP5) in a variable method:

Make an array of two entries where the 0th entry is the name of the class to be invoked ('self' and 'parent' work as well) and the 1st entry is the name of the function.  Basically, a 'callback' variable is either a string (the name of the function) or an array (0 => 'className', 1 => 'functionName').

Then, to call that function, you can use either call_user_func() or call_user_func_array().  Examples:

<?php
class A {

  protected $a;
  protected $c;

  function __construct() {
   $this->a = array('self', 'a');
   $this->c = array('self', 'c');
  }

  static function a($name, &$value) {
   echo $name,' => ',$value++,"\n";
  }

  function b($name, &$value) {
   call_user_func_array($this->a, array($name, &$value));
  }

  static function c($str) {
   echo $str,"\n";
  }

  function d() {
   call_user_func_array($this->c, func_get_args());
  }

  function e() {
   call_user_func($this->c, func_get_arg(0));
  }

}

class B extends A {

  function __construct() {
   $this->a = array('parent', 'a');
   $this->c = array('self', 'c');
  }

  static function c() {
   print_r(func_get_args());
  }

  function d() {
   call_user_func_array($this->c, func_get_args());
  }

  function e() {
   call_user_func($this->c, func_get_args());
  }

}

$a =& new A;
$b =& new B;
$i = 0;

A::a('index', $i);
$a->b('index', $i);

$a->c('string');
$a->d('string');
$a->e('string');

# etc.
?>
Storm
03-May-2005 11:34
This can quite useful for a dynamic database class:

(Note: This just a simplified section)

<?php
class db {

   private $host = 'localhost';
   private $user = 'username';
   private $pass = 'password';
   private $type = 'mysqli';
  
   public $lid = 0;

   // Connection function
   function connect() {
       $connect = $this->type.'_connect';
          
       if (!$this->lid = $connect($this->host, $this->user, $this->pass)) {
           die('Unable to connect.');
       }
 }
}
$db  = new db;
$db->connect();
?>

Much easier than having multiple database classes or even extending a base class.
ian at NO_SPAM dot verteron dot net
20-Dec-2002 11:33
A good method to pass around variables containing function names within some class is to use the same method as the developers use in preg_replace_callback - with arrays containing an instance of the class and the function name itself.

function call_within_an_object($fun)
{
  if(is_array($fun))
  {
   /* call a function within an object */
   $fun[0]->{$fun[1]}();
  }
  else
  {
   /* call some other function */
   $fun();
  }
}

function some_other_fun()
{
  /* code */
}

class x
{
  function fun($value)
  {
   /* some code */
  }
}

$x = new x();

/* the following line calls $x->fun() */
call_within_an_object(Array($x, 'fun'));

/* the following line calls some_other_fun() */
call_within_an_object('some_other_fun');
madeinlisboa at yahoo dot com
05-Sep-2002 08:14
Finally, a very easy way to call a variable method in a class:

Example of a class:

class Print() {
   var $mPrintFunction;

   function Print($where_to) {
       $this->mPrintFunction = "PrintTo$where_to";
   }

   function PrintToScreen($content) {
       echo $content;
   }

   function PrintToFile($content) {
       fputs ($file, $contents);
   }

.. .. ..

   // first, function name is parsed, then function is called
   $this->{$this->mPrintFunction}("something to print");
}
msmith at pmcc dot com
03-May-2002 07:49
Try the call_user_func() function.  I find it's a bit simpler to implement, and at very least makes your code a bit more readable... much more readable and simpler to research for someone who isn't familiar with this construct.
anpatel at NOSPAM_cbu dot edu
18-Mar-2002 01:11
Yes interpolation can be very tricky. I suggest that you always use parenthesis, or curly brackets(whichever applies) to make your expression clear.

Dont ever depend on a language's expression parse preference order.
retro at enx dot org
14-Jan-2002 11:18
Another way to have php parse a variable within an object as a function is to simply set a temporary variable to its value. For example:

$obj->myfunction = "foo";
$x = $obj->myfunction;
$x(); // calls the function named "foo"
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