From http://git.php.net/?p=php-src.git;a=blob;f=ext/standard/array.c;h=94c5e7e;hb=refs/heads/PHP-5.4#l306

array.c

/* {{{ proto int count(mixed var [, int mode])
   Count the number of elements in a variable (usually an array) */
PHP_FUNCTION(count)
{
	zval *array;
	long mode = COUNT_NORMAL;

	if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "z|l", &array, &mode) == FAILURE) {
		return;
	}

	switch (Z_TYPE_P(array)) {
		case IS_NULL:
			RETURN_LONG(0);
			break;
		case IS_ARRAY:
			RETURN_LONG (php_count_recursive (array, mode TSRMLS_CC));
			break;
		case IS_OBJECT: {
#ifdef HAVE_SPL
			zval *retval;
#endif
			/* first, we check if the handler is defined */
			if (Z_OBJ_HT_P(array)->count_elements) {
				RETVAL_LONG(1);
				if (SUCCESS == Z_OBJ_HT(*array)->count_elements(array, &Z_LVAL_P(return_value) TSRMLS_CC)) {
					return;
				}
			}
#ifdef HAVE_SPL
			/* if not and the object implements Countable we call its count() method */
			if (Z_OBJ_HT_P(array)->get_class_entry && instanceof_function(Z_OBJCE_P(array), spl_ce_Countable TSRMLS_CC)) {
				zend_call_method_with_0_params(&array, NULL, NULL, "count", &retval);
				if (retval) {
					convert_to_long_ex(&retval);
					RETVAL_LONG(Z_LVAL_P(retval));
					zval_ptr_dtor(&retval);
				}
				return;
			}
#endif
		}
		default:
			RETURN_LONG(1);
			break;
	}
}
/* }}} */