Sessions getting lost

[sylvest]

I'm having a problem on a website that I develop. It seems that sometimes when I call session_start(), it fails to find a previously opened session, and opens a new one, which means that I lose all my session variables. Here's a section of log:

[27-Feb-2020 17:39:43.496 GMT 0] on localhost,  L_TRACE MemberId 18 attempted to login with PasswordStatus = 4                           at line 169   in file: /home/sylvest/public_html/swchoir/do_login.php
[27-Feb-2020 17:39:43.497 GMT 0] on localhost,  L_TRACE Session ID = qdgs4obtlicrlcgnp6tc179e87, $_SESSION = {"breadcrumbs":[{"shortname":"Login", "filename":"index.php"}], "request_index":2, "test_mode":0, "username_entered":"sasb", "login_member":null, "login_memberid":"18", "login_name":"Sue
Sylvester-Bradley", "auth_level":0, "return_page":"\/swchoir\/do_login.php", "edit_member_msg":"Your password has expired,  and you need to choose a new one before you can access these pages. Please
do that now.", "edit_memberid":"18", "edit_member_mode":"change_password"} at line 319 in file: /home/sylvest/public_html/swchoir/do_login.php
[27-Feb-2020 17:39:43.497 GMT 0] on localhost,  L_TRACE Send cookie username_entered:sasb succeeded                                      at line 334   in file: /home/sylvest/public_html/swchoir/do_login.php
[27-Feb-2020 17:39:43.497 GMT 0] on localhost,  L_TRACE $to_page = edit_member.php?v=1582819873                                          at line 1200  in file: /home/sylvest/include/functions.php
    Called from redirect(edit_member.php?v=1582819873) at line 339 in file /home/sylvest/public_html/swchoir/do_login.php
[27-Feb-2020 17:39:43.497 GMT 0] on localhost,  L_TRACE Redirecting to Location: https://sylvesterbradley.org/swchoir/edit_member.php?v=1582819873 using header at line 1201 in file: /home/sylvest/include/functions.php
    Called from redirect(edit_member.php?v=1582819873) at line 339 in file /home/sylvest/public_html/swchoir/do_login.php
[27-Feb-2020 17:39:43.817 GMT 0] on  L_TRACE Started file                                                                                at line 37    in file: /home/sylvest/public_html/swchoir/edit_member.php
[27-Feb-2020 17:39:43.817 GMT 0] on  L_TRACE Started file                                                                                at line 5     in file: /home/sylvest/include/basic_init.php
[27-Feb-2020 17:39:43.817 GMT 0] on  L_TRACE Started file                                                                                at line 4     in file: /home/sylvest/include/functions.php
[27-Feb-2020 17:39:43.817 GMT 0] on  L_TRACE Started file                                                                                at line 13    in file: /home/sylvest/include/log_functions.php
[27-Feb-2020 17:39:43.818 GMT 0] on ,  L_TRACE Resumed file                                                                              at line 9     in file: /home/sylvest/include/functions.php
    Called from require_once(/home/sylvest/include/functions.php) at line 129 in file /home/sylvest/include/basic_init.php
    Called from require(/home/sylvest/include/basic_init.php) at line 128 in file /home/sylvest/public_html/swchoir/edit_member.php
[27-Feb-2020 17:39:43.818 GMT 0] on ,  L_TRACE Resumed file                                                                              at line 140   in file: /home/sylvest/include/basic_init.php
    Called from require(/home/sylvest/include/basic_init.php) at line 128 in file /home/sylvest/public_html/swchoir/edit_member.php
[27-Feb-2020 17:39:43.818 GMT 0] on ,  L_TRACE session_status() = PHP_SESSION_NONE                                                       at line 782   in file: /home/sylvest/include/functions.php
    Called from my_session_start() at line 152 in file /home/sylvest/include/basic_init.php
    Called from require(/home/sylvest/include/basic_init.php) at line 128 in file /home/sylvest/public_html/swchoir/edit_member.php
[27-Feb-2020 17:39:43.818 GMT 0] on ,  L_TRACE No session is active                                                                      at line 789   in file: /home/sylvest/include/functions.php
    Called from my_session_start() at line 152 in file /home/sylvest/include/basic_init.php
    Called from require(/home/sylvest/include/basic_init.php) at line 128 in file /home/sylvest/public_html/swchoir/edit_member.php
[27-Feb-2020 17:39:43.819 GMT 0] on ,  L_TRACE Session ID 5ks74rcukqqbkjdhchiru310q3 is now active, RemoteIp = 90.251.34.221             at line 804   in file: /home/sylvest/include/functions.php
    Called from my_session_start() at line 152 in file /home/sylvest/include/basic_init.php
    Called from require(/home/sylvest/include/basic_init.php) at line 128 in file /home/sylvest/public_html/swchoir/edit_member.php
[27-Feb-2020 17:39:43.819 GMT 0] on ,  L_TRACE Session ID = 5ks74rcukqqbkjdhchiru310q3, $_SESSION = []                                   at line 805   in file: /home/sylvest/include/functions.php
    Called from my_session_start() at line 152 in file /home/sylvest/include/basic_init.php
    Called from require(/home/sylvest/include/basic_init.php) at line 128 in file /home/sylvest/public_html/swchoir/edit_member.php

In the first bolded section, you can see that session ID qdgs4obtlicrlcgnp6tc179e87 is active, and that there are all sorts of session variables set.

In the second bolded session, session_status() returned PHP_SESSION_NONE, so when I called session_start() it created a new session 5ks74rcukqqbkjdhchiru310q3 and all my session variables have gone.   

What am I doing that is causing the session to be lost? How do I make sure that when I call session_start() it retrieves any previous session if there is one, and does not create a new one? Can I implement my own version of session_start() and thus see exactly what it is doing and put in logic to prevent this error? What exactly does session_start() do?

Thank you - Rowan                                          ,