Objects comparison in PHP

Question

SO,

The problem

It\'s not well-known, but PHP allows to compare objects - and not just on equality == - but on <

Answer 1

Each class in php has an associated structure (in the c code) of handler functions, it looks like

struct _zend_object_handlers {
    /* general object functions */
    zend_object_add_ref_t                   add_ref;
    zend_object_del_ref_t                   del_ref;
[...]
    zend_object_compare_t                   compare_objects;
[...]
};

compare_objects points to a function that "takes two objects" and returns -1,0,1 according to whatever this comparator defines as the order (just like strcmp() does for strings).
This function is used only when both operands (objects) point to the same comparision function - but let's just stick with this case.
That's where e.g. DateTime "adds" its feature to compare two DateTime instances, it just defines another, DateTime-specific compare_objects function and puts it in the structure describing its class.

static void date_register_classes(TSRMLS_D)
{
[...]

    INIT_CLASS_ENTRY(ce_date, "DateTime", date_funcs_date);
    ce_date.create_object = date_object_new_date;
[...]
    date_object_handlers_date.compare_objects = date_object_compare_date;

So if you want to know (exactly) how two DateTime instances are compared, take a look at date_object_compare_date.

The comparision described in the manual (at least for the case cmp(o1,o2)==0) seems to be implemented in zend_std_compare_objects. And it's used by both StdClass and a simple user defined class like e.g.

 $b;

But other classes (in php extensions) do set other functions. DateTime, ArrayObject, PDOStatement, even Closures use different functions.
But I haven't found a way to define a comparision function/method in script code (but haven't looked too hard/long)