PHP函数名参数和闭包

array array_filter ( array $input [, callable $callback = "" ] )

<?php 
// An example callback function
function my_callback_function() {
    echo 'hello world!';
}
// An example callback method
class MyClass {
    static function myCallbackMethod() {
        echo 'Hello World!';
    }
}
// Type 1: Simple callback
call_user_func('my_callback_function'); 
// Type 2: Static class method call
call_user_func(array('MyClass', 'myCallbackMethod')); 
// Type 3: Object method call
$obj = new MyClass();
call_user_func(array($obj, 'myCallbackMethod'));
// Type 4: Static class method call (As of PHP 5.2.3)
call_user_func('MyClass::myCallbackMethod');
// Type 5: Relative static class method call (As of PHP 5.3.0)
class A {
    public static function who() {
        echo "A\n";
    }
}
class B extends A {
    public static function who() {
        echo "B\n";
    }
}
call_user_func(array('B', 'parent::who')); // A
?>

<?php
$input = array_flip( range( 'a', 'z' ) );
$consonants = array_filter_key( $arr, function( $elem ) {
    $vowels = "aeiou";
    return strpos( $vowels, strtolower( $elem ) ) === false;
} );
?>