PHPAPI void php_session_start(TSRMLS_D) /* {{{ */
{
	zval **ppid;
	zval **data;
	char *p, *value;
	int nrand;
	int lensess;

	if (PS(use_only_cookies)) {
		PS(apply_trans_sid) = 0;
	} else {
		PS(apply_trans_sid) = PS(use_trans_sid);
	}

	switch (PS(session_status)) {
		case php_session_active:
			php_error(E_NOTICE, "A session had already been started - ignoring session_start()");
			return;
			break;

		case php_session_disabled:
			value = zend_ini_string("session.save_handler", sizeof("session.save_handler"), 0);
			if (!PS(mod) && value) {
				PS(mod) = _php_find_ps_module(value TSRMLS_CC);
				if (!PS(mod)) {
					php_error_docref(NULL TSRMLS_CC, E_WARNING, "Cannot find save handler '%s' - session startup failed", value);
					return;
				}
			}
			value = zend_ini_string("session.serialize_handler", sizeof("session.serialize_handler"), 0);
			if (!PS(serializer) && value) {
				PS(serializer) = _php_find_ps_serializer(value TSRMLS_CC);
				if (!PS(serializer)) {
					php_error_docref(NULL TSRMLS_CC, E_WARNING, "Cannot find serialization handler '%s' - session startup failed", value);
					return;
				}
			}
			PS(session_status) = php_session_none;
			/* fallthrough */

		default:
		case php_session_none:
			PS(define_sid) = 1;
			PS(send_cookie) = 1;
	}

	lensess = strlen(PS(session_name));

	/* Cookies are preferred, because initially
	 * cookie and get variables will be available. */

	if (!PS(id)) {
		if (PS(use_cookies) && zend_hash_find(&EG(symbol_table), "_COOKIE", sizeof("_COOKIE"), (void **) &data) == SUCCESS &&
				Z_TYPE_PP(data) == IS_ARRAY &&
				zend_hash_find(Z_ARRVAL_PP(data), PS(session_name), lensess + 1, (void **) &ppid) == SUCCESS
		) {
			ppid2sid(ppid TSRMLS_CC);
			PS(apply_trans_sid) = 0;
			PS(define_sid) = 0;
		}

		if (!PS(use_only_cookies) && !PS(id) &&
				zend_hash_find(&EG(symbol_table), "_GET", sizeof("_GET"), (void **) &data) == SUCCESS &&
				Z_TYPE_PP(data) == IS_ARRAY &&
				zend_hash_find(Z_ARRVAL_PP(data), PS(session_name), lensess + 1, (void **) &ppid) == SUCCESS
		) {
			ppid2sid(ppid TSRMLS_CC);
		}

		if (!PS(use_only_cookies) && !PS(id) &&
				zend_hash_find(&EG(symbol_table), "_POST", sizeof("_POST"), (void **) &data) == SUCCESS &&
				Z_TYPE_PP(data) == IS_ARRAY &&
				zend_hash_find(Z_ARRVAL_PP(data), PS(session_name), lensess + 1, (void **) &ppid) == SUCCESS
		) {
			ppid2sid(ppid TSRMLS_CC);
		}
	}

	/* Check the REQUEST_URI symbol for a string of the form
	 * '<session-name>=<session-id>' to allow URLs of the form
	 * http://yoursite/<session-name>=<session-id>/script.php */

	if (!PS(use_only_cookies) && !PS(id) && PG(http_globals)[TRACK_VARS_SERVER] &&
			zend_hash_find(Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_SERVER]), "REQUEST_URI", sizeof("REQUEST_URI"), (void **) &data) == SUCCESS &&
			Z_TYPE_PP(data) == IS_STRING &&
			(p = strstr(Z_STRVAL_PP(data), PS(session_name))) &&
			p[lensess] == '='
	) {
		char *q;

		p += lensess + 1;
		if ((q = strpbrk(p, "/?\\"))) {
			PS(id) = estrndup(p, q - p);
			PS(send_cookie) = 0;
		}
	}

	/* Check whether the current request was referred to by
	 * an external site which invalidates the previously found id. */

	if (PS(id) &&
			PS(extern_referer_chk)[0] != '\0' &&
			PG(http_globals)[TRACK_VARS_SERVER] &&
			zend_hash_find(Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_SERVER]), "HTTP_REFERER", sizeof("HTTP_REFERER"), (void **) &data) == SUCCESS &&
			Z_TYPE_PP(data) == IS_STRING &&
			Z_STRLEN_PP(data) != 0 &&
			strstr(Z_STRVAL_PP(data), PS(extern_referer_chk)) == NULL
	) {
		efree(PS(id));
		PS(id) = NULL;
		PS(send_cookie) = 1;
		if (PS(use_trans_sid) && !PS(use_only_cookies)) {
			PS(apply_trans_sid) = 1;
		}
	}

	/* Finally check session id for dangarous characters
	 * Security note: session id may be embedded in HTML pages.*/
	if (PS(id) && strpbrk(PS(id), "\r\n\t <>'\"\\")) {
		efree(PS(id));
		PS(id) = NULL;
	}

	php_session_initialize(TSRMLS_C);
	php_session_cache_limiter(TSRMLS_C);

	if ((PS(mod_data) || PS(mod_user_implemented)) && PS(gc_probability) > 0) {
		int nrdels = -1;

		nrand = (int) ((float) PS(gc_divisor) * php_combined_lcg(TSRMLS_C));
		if (nrand < PS(gc_probability)) {
			PS(mod)->s_gc(&PS(mod_data), PS(gc_maxlifetime), &nrdels TSRMLS_CC);
#ifdef SESSION_DEBUG
			if (nrdels != -1) {
				php_error_docref(NULL TSRMLS_CC, E_NOTICE, "purged %d expired session objects", nrdels);
			}
#endif
		}
	}
}
/* }}} */

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.