PHP supports the concept of variable functions. This means that if a variable name has parentheses appended to it, PHP will look for a function with the same name as whatever the variable evaluates to, and will attempt to execute it. Among other things, this can be used to implement callbacks, function tables, and so forth.
Variable functions won't work with language constructs such as echo, print, unset(), isset(), empty(), include, require and the like. Utilize wrapper functions to make use of any of these constructs as variable functions.
Example #1 Variable function example
<?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()
?>
Object methods can also be called with the variable functions syntax.
Example #2 Variable method example
<?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()
?>
When calling static methods, the function call is stronger than the static property operator:
Example #3 Variable method example with static properties
<?php
class Foo
{
static $variable = 'static property';
static function Variable()
{
echo 'Method Variable called';
}
}
echo Foo::$variable; // This prints 'static property'. It does need a $variable in this scope.
$variable = "Variable";
Foo::$variable(); // This calls $foo->Variable() reading $variable in this scope.
?>
As of PHP 5.4.0, you can call any callable stored in a variable.
Example #4 Complex callables
class Foo
{
static function bar()
{
echo "bar\n";
}
function baz()
{
echo "baz\n";
}
}
$func = array("Foo", "bar");
$func(); // prints "bar"
$f = array(new Foo, "baz");
$func(); // prints "baz"
$f = "Foo::bar";
$func(); // prints "bar" as of PHP 7.0.0; prior, it raised a fatal error
See also is_callable(), call_user_func(), variable variables and function_exists().
| Version | Description |
|---|---|
| 7.0.0 | 'ClassName::methodName' is allowed as variable function. |
| 5.4.0 | Arrays, which are valid callables, are allowed as variable functions. |
i'm not sure, but simple mistake in this place ($f instead $func):
<?php
$func = array("Foo", "bar");
$func(); // prints "bar"
$f = array(new Foo, "baz");
$func(); // prints "baz"
$f = "Foo::bar";
$func(); // prints "bar" as of PHP 7.0.0; prior, it raised a fatal error
?>
$ wget http://www.php.net/get/php_manual_en.tar.gz/from/a/mirror
$ grep -l "\$\.\.\." php-chunked-xhtml/function.*.html
List of functions that accept variable arguments.
<?php
array_diff_assoc()
array_diff_key()
array_diff_uassoc()
array()
array_intersect_ukey()
array_map()
array_merge()
array_merge_recursive()
array_multisort()
array_push()
array_replace()
array_replace_recursive()
array_unshift()
call_user_func()
call_user_method()
compact()
dba_open()
dba_popen()
echo()
forward_static_call()
fprintf()
fscanf()
httprequestpool_construct()
ibase_execute()
ibase_set_event_handler()
ibase_wait_event()
isset()
list()
maxdb_stmt_bind_param()
maxdb_stmt_bind_result()
mb_convert_variables()
newt_checkbox_tree_add_item()
newt_grid_h_close_stacked()
newt_grid_h_stacked()
newt_grid_v_close_stacked()
newt_grid_v_stacked()
newt_win_choice()
newt_win_entries()
newt_win_menu()
newt_win_message()
newt_win_ternary()
pack()
printf()
register_shutdown_function()
register_tick_function()
session_register()
setlocale()
sprintf()
sscanf()
unset()
var_dump()
w32api_deftype()
w32api_init_dtype()
w32api_invoke_function()
wddx_add_vars()
wddx_serialize_vars()
?>
A small, but helpful note. If you are trying to call a static function from a different namespace, you must use the fully qualified namespace, even if they have the same top level namespace(s). For example if you have the following class to call:
<?php
namespace Project\TestClass;
class Test {
static function funcToCall() {
return "test";
}
}
?>
You must call it as:
<?php
namespace Project\OtherTestClass;
class OtherTest {
static function callOtherFunc() {
$func = '\Project\TestClass::funcToCall';
$func();
}
}
?>
and not:
<?php
class OtherTest {
static function callOtherFunc() {
$func = 'TestClass::funcToCall';
$func();
}
}
?>
A good method to pass around variables containing function names within some class is to use the same method as the developers use in preg_replace_callback - with arrays containing an instance of the class and the function name itself.
function call_within_an_object($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');
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");
}
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.
?>
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.
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.
Variable functions allows higher-order programming.
Here is the classical map example.
<?php
/*
* Map function. At each $element of the $list, calls $fun([$arg1,[$arg2,[...,]],$element,$accumulator),
* stores the return value into $accumulator for the next loop. Returns the last return value of the function,
*
* Notes : uses call_user_func_array() so passing parameters doesn't depend on $fun signature
* It also returns FALSE upon error.
* Please check the php documentation for more information
*/
function map($fun, $list,$params=array()){
$acc=NULL;
$last=array_push($params, NULL,$acc)-1; // alloc $element and $acc at the end
foreach($list as $params[$last-1]){
$params[$last]=call_user_func_array($fun , $params );
}
$acc=array_pop($params);
return $acc;
}
function add($element,$acc){ // maybe only with multi-length function
if ($acc == NULL);
return $acc=$element+$acc;
}
$result=0;
$result=addTo($result,1);
$result=addTo($result,2);
$result=addTo($result,3);
echo "result = $result\n";
$result=0;
$result=map('addTo',array(1,2,3));
echo "result= $result\n";
?>
Create and call a dynamically named function
<?php
$tmp = "foo";
$$tmp = function() {
global $tmp;
echo $tmp;
};
$$tmp();
?>
Outputs "foo"
