<?php

class MyCollection extends MongoCollection
{
    public static function toIndexString($a)
    {
        return parent::toIndexString($a);
    }
}
var_dump(MyCollection::toIndexString('x'));
var_dump(MyCollection::toIndexString('x.y.z'));
var_dump(MyCollection::toIndexString('x_y.z'));
var_dump(MyCollection::toIndexString(array('x' => 1)));
var_dump(MyCollection::toIndexString(array('x' => -1)));
var_dump(MyCollection::toIndexString(array('x' => 1, 'y' => -1)));
<?php

class MyCollection extends MongoCollection
{
    public static function toIndexString($a)
    {
        return parent::toIndexString($a);
    }
}
var_dump(MyCollection::toIndexString(null));