Description:
------------
The PHP core function "is_callable" invokes the autoloading when receives a string callback that has "::", even when the class has not a valid name.

It could be smarter invoking autoloading only when the class name is a valid class name, as described by the expression [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]* from documentation (http://www.php.net/manual/en/language.oop5.basic.php). It should check whether a namespace was used too.

Often, is_callable function is not invoked in a try/catch context. So, in many cases, it could crash PHP execution. For example, in a dispatcher implementation that receives a controller class and an action (method) by user, create a callback and test it with is_callable.

Test script:
---------------
<?php

function __autoload($class) { 
    printf("Required class: '%s'\n", $class);
    throw new Exception('Error');
}

var_dump(is_callable('invalid name::method'));

Expected result:
----------------
bool(false)

Actual result:
--------------
Required class: 'invalid name'
PHP Fatal error:  Uncaught exception 'Exception' with message 'Error' in %s:5