- Have __toString() be called if available in all places an object is used

- Have __toString() be called if available in all places an object is used
  as a string.
#
# Note that "Object #<id>" is no longer afallback for debugging purpose use
# var_dump, which was made for debugging. If you used this to grab the id
# of an object you can never rely on this. For object storage look at SPL's
# ObjectStorage class.
#
# Note the signature change in the cast handler:
#
# int (*cast_t)(zval *readobj, zval *writeobj, int type, int should_free TSRMLS_DC);
# int (*cast_t)(zval *readobj, zval *retval, int type TSRMLS_DC);