Specifying the datatype is not really needed unless your passing in classes / object but if i was do do that i would create a data type class for each datatype

PHP Code: 
class String implements Multilingual
{
     public $value;
     function __construct($string)
     {
         $this->value = $string;
     }
     function __toString($string)
     {
          return $this->value;
     }

     function __call($method,$args)
     {
          if(function_exists($method))
          {
              $this->value = $method($this->value);
          }
          return $this;
     }
}
$myString = new String("Hello World");
$myString->trim(); 
PHP Code: 
class Array implements Iterator
{
    //...methods
} 
Then within my user interfaces i can specifically expect a String or an Array type.