You have an error in your SQL syntax; check the manual that corresponds to your mariadb server version for the right syntax

We can't help you: we can't see your code, or access your database - and you probably need both in order to work out what is wrong.

So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. How you use it depends on your compiler system, but a quick Google should give you the info you need: php debugger - Google Search[^]

Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.

Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!

UPDATE SQL Syntax for normal cases is like

SQL

UPDATE <table_name>
SET <column_name> = <value>
[WHERE <condition> ]

Your update SQL

SQL

UPDATE signal100 SET signal100 = CASE WHEN @num > 0 THEN signal100 ELSE 0 END

is Clearly Erroneous because your table name and column name are same.

create table signal100(signal100int);
insert into signal100(signal100) values (1),(2),(3);
update signal100 set signal100= 4 where signal100= 2;

The error message suggest that you check the documentation: UPDATE - MariaDB Knowledge Base[^]

Now that you have finally shared your code I can tell you that you have omitted a semi-colon after END.

dbquery("SET @num = (SELECT COUNT(*) AS num FROM dispatchersonduty); UPDATE signal100 SET signal100 = CASE WHEN @num > 0 THEN signal100 ELSE 0 END;", false);