Why not using namespaced functions instead of static methods all in the same class ? This would have allowed to add new functions. This would even have allowed to use the _ namespace.

With namespaces:

    namespace _;  
    function each() {
    }

    // chaining can be achieved with this:

    class Wrapper {
        private $coll;
        function __construct($coll) {
            $this->coll = $coll;
        }
        function __call($name, $args) {
            array_unshift($this->coll,$args);
            return new self(call_user_func_array('_\\'.$name, $args));
        }
    }

    function __($coll) {
        return new Wrapper($coll);
    }

    use _;  
    _\each(...);

    // or
  
    __($coll)->each(...);

    // I can add new functions

    namespace _;
    function something(){}

With static methods:

    class __ {  
        function each() {  
        }  
    }

    // throws strict errors

    __::each(...);

    // can't add functions

Also it seems that the normal way to use the library is to call isntance methods statically, which triggers many warnings with E_STRICT :(

https://github.com/lstrojny/functional-php also brings some functional stuff to php.