How do I dynamically invoke a class method in PHP?

Option 1

// invoke an instance method
$instance = new Instance();
$instanceMethod = 'bar';
$instance->$instanceMethod();

// invoke a static method
$class = 'NameOfTheClass';
$staticMethod = 'blah';
$class::$staticMethod();

Option 2

// invoke an instance method
$instance = new Instance();
call_user_func( array( $instance, 'method' ) );

// invoke a static method
$class = 'NameOfTheClass';
call_user_func( array( $class, 'nameOfStaticMethod' ) );
call_user_func( 'NameOfTheClass::nameOfStaticMethod' ); // (As of PHP 5.2.3)

Option 1 is faster than Option 2 so try to use them unless you don't know how many arguments your going to be passing to the method.

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Edit: Previous editor did great job of cleaning up my answer but removed mention of call_user_func_array which is different then call_user_func.

PHP has

mixed call_user_func ( callable $callback [, mixed $parameter [, mixed $... ]] ) 

http://php.net/manual/en/function.call-user-func.php

AND

mixed call_user_func_array ( callable $callback , array $param_arr )

http://php.net/manual/en/function.call-user-func-array.php

Using call_user_func_array is orders of magnitude slower then using either option listed above.

It works both ways - you need to use the right syntax

//  Non static call
call_user_func( array( $obj, 'method' ) );

//  Static calls
call_user_func( array( 'ClassName', 'method' ) );
call_user_func( 'ClassName::method' ); // (As of PHP 5.2.3)

You mean like this?

<?php

class A {
    function test() {
        print 'test';
    }
}

$function = 'test';

// method 1
A::$function();

// method 2
$a = new A;    
$a->$function();

?>