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PHP : Language Reference : Functions : Variable functions

Variable functions

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. You need to use your own wrapper function to utilize any of these constructs as variable functions.

Example 8.14. Variable function example

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<?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()
?>


You can also call an object's method by using the variable functions feature.

Example 8.15. Variable method example

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<?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()

?>


See also call_user_func(), variable variables and function_exists().

Code Examples / Notes » functions.variable_functions

anpatel
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.

msmith
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.

storm
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.

mike
It may be useful to note that 'variable functions' can only be used in conjunction with functions, and not language constructs such as echo, die, exit, etc.

boards
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.
?>

madeinlisboa
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");
}

retro
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"

ian
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');

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User-defined functions
Function arguments
Returning values
Variable functions
Internal (built-in) functions
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