If you are talking about native PHP support for object oriented programming, then PHP4 does have some support as discussed in the PHP Manual's entry on Classes and Objects (PHP 4), with more support provided in PHP5 as you can read in Classes and Objects (PHP 5).
If you are talking about interfaces as more than just the general term but more like what Java provides with its interface keyword, then you might want to know that it is available in PHP5: Object Interfaces.