You can use the fallback policy to provide mocks for built-in functions like time(). You therefore have to call those functions unqualified:
<?php
namespace foo;
function time() {
return 1234;
}
assert (1234 == time());
?>
However there's a restriction that you have to define the mock function before the first usage in the tested class method. This is documented in Bug #68541.
You can find the mock library php-mock at GitHub.