(PHP 7 >= 7.2.0)
spl_object_id — Return the integer object handle for given object
$obj) : int
This function returns a unique identifier for the object. The object id is unique for the lifetime of the object. Once the object is destroyed, its id may be reused for other objects. This behavior is similar to spl_object_hash().
An integer identifier that is unique for each currently existing object and is always the same for each object.
Ejemplo #1 A spl_object_id() example
$id = spl_object_id($object);
$storage[$id] = $object;
When an object is destroyed, its id may be reused for other objects.