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I am trying to create a trigger on updating the salary field of this table.
customers(ID, Name, Age, Address, Salary) from this site SQL Trigger
While creating it shows the following error

MySQL said:
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'OR INSERT OR UPDATE ON customers
FOR EACH ROW
WHEN (NEW.ID > 0)
DECLARE
' at line 2

Here is the code snippet:

BEFORE DELETE OR INSERT OR UPDATE ON customers 
FOR EACH ROW 
WHEN (NEW.ID > 0) 
DECLARE 
 sal_diff number; 
BEGIN `enter code here`
 sal_diff := :NEW.salary - :OLD.salary; 
 dbms_output.put_line('Old salary: ' || :OLD.salary); 
 dbms_output.put_line('New salary: ' || :NEW.salary); 
 dbms_output.put_line('Salary difference: ' || sal_diff); 
END

Note that, I'm using phpMyAdmin. Does dbms_ouput.put_line() works there?

As said in the comments, you are using Oracle syntax, it can't work on MySQL.

Then, there's no reason to have this trigger on DELETE and INSERT because your aim is to compute the salary difference on a specific row before and after it get updated. You can't have a value NEW on DELETE, and you can't have a value OLD on INSERT.
Thus your computation is only meaningful on UPDATE.

Here's the correct MySQL syntax (I am assuming that you have a column sal_diff on your table)

DELIMITER $$
CREATE TRIGGER `update_sal_diff`
BEFORE UPDATE ON `customers `
FOR EACH ROW
BEGIN
 SET NEW.sal_diff = NEW.salary - OLD.salary;
END $$
DELIMITER ;

If this is not what you are trying to achieve, edit your question and add clear requirements