From Get docs

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. Utilize wrapper functions to make use of any of these constructs as variable functions.

Example #1 Variable function example

<?phpfunction foo() {    echo "In foo()<br />\n";}function bar($arg = ){    echo "In bar(); argument was '$arg'.<br />\n";}// This is a wrapper function around echofunction 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

<?phpclass 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

<?phpclass 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.?>

Example #4 Complex callables

<?phpclass Foo{    static function bar()    {        echo "bar\n";    }    function baz()    {        echo "baz\n";    }}$func = array("Foo", "bar");$func(); // prints "bar"$func = array(new Foo, "baz");$func(); // prints "baz"$func = "Foo::bar";$func(); // prints "bar"?>