This KB article describe the nature of the logs:

re0 jweb[72189]: MGD: xml header: done reading XML header
re0 jweb[72189]: No Fatal error encountered. Shutdown-function called when script exec ends.
re0 php[72195]: CACHING FLOW: sajax_handle_client_request returning as no rs args -- Not old page request.. :)

Logs is observed on EX4300-48-T & EX9204

Upon checking this behaviour, It seems those log messages are generated when someone or somebody constantly reloads the WebGUI (Jweb) login page for that device, without even login into.
root@SW> start shell
root@SW:RE:0% cd /var/jail/sess/
root@SW:RE:0% ls -l
total 4
-rw-r--r-- 1 nobody wheel 333 Jul 26 18:07 php.log

root@SW:RE:0% ls -l
total 4
-rw-r--r-- 1 nobody wheel 666 Jul 26 18:07 php.log

root@SW:RE:0% cat php.log
[26-Jul-2022 18:06:51 America/Montreal] MGD: xml header: done reading XML header
[26-Jul-2022 18:06:51 America/Montreal] No Fatal error encountered. Shutdown-function called when script exec ends.
[26-Jul-2022 18:07:36 America/Montreal] CACHING FLOW: sajax_handle_client_request returning as no rs args -- Not old page request.. :)
[26-Jul-2022 18:07:36 America/Montreal] MGD: xml header: done reading XML header
[26-Jul-2022 18:07:36 America/Montreal] No Fatal error encountered. Shutdown-function called when script exec ends.
[26-Jul-2022 18:09:47 America/Montreal] CACHING FLOW: sajax_handle_client_request returning as no rs args -- Not old page request.. :)

When you log in, a different output is shown and much more information is contained on that php.log file. This is logged when the session is finished or logged out and the login page is called out again:
[26-Jul-2022 18:16:11 America/Montreal] PERF: sajax_logout Start: 2022-07-26 18:16:11.129567
[26-Jul-2022 18:16:11 America/Montreal] CACHING FLOW: query junoscript query.. rpc = request-web-management-logout
[26-Jul-2022 18:16:11 America/Montreal] CACHING FLOW: query make query complete... Query = <request-web-management-logout><session-id>94cd98a7d97a6d4904b1617405c4d98cc44577d1</session-id></request-web-management-logout>
[26-Jul-2022 18:16:11 America/Montreal] PERF: sajax_logout End: 2022-07-26 18:16:11.144007
[26-Jul-2022 18:16:11 America/Montreal] No Fatal error encountered. Shutdown-function called when script exec ends.
[26-Jul-2022 18:16:11 America/Montreal] CACHING FLOW: sajax_handle_client_request returning as no rs args -- Not old page request... :)

Please check that you don’t have any automated system constantly trying to log in   to JWEB, or it could be one or multiple connections stuck:
>show system connections | match 443

If you find any, please delete/deactivate JWEB configuration, commit, and reapply it. After this, please double-check, if php.log is still increasing.

2024-03-01 : Article Created

2025-7-30 :  Improved formatting

2025-8-12 : Replaced hostname,  filled in Modification History