static int php_session_destroy(TSRMLS_D) /* {{{ */
{
	int retval = SUCCESS;

	if (PS(session_status) != php_session_active) {
		php_error_docref(NULL TSRMLS_CC, E_WARNING, "Trying to destroy uninitialized session");
		return FAILURE;
	}

	if (PS(id) && PS(mod)->s_destroy(&PS(mod_data), PS(id) TSRMLS_CC) == FAILURE) {
		retval = FAILURE;
		php_error_docref(NULL TSRMLS_CC, E_WARNING, "Session object destruction failed");
	}

	php_rshutdown_session_globals(TSRMLS_C);
	php_rinit_session_globals(TSRMLS_C);

	return retval;
}
/* }}} */

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.