To get all the server variables run this query either in the SQL window of your preferred interface (PHPMyAdmin or other) or in the MySQL CLI interface
SHOW VARIABLES;
You can specify if you want the session variables or the global variables as follows:
Session variables:
SHOW SESSION VARIABLES;
Global variables:
SHOW GLOBAL VARIABLES;
Like any other SQL command you can add parameters to your query such as the LIKE command:
SHOW [GLOBAL | SESSION] VARIABLES LIKE 'max_join_size';
Or, using wildcards:
SHOW [GLOBAL | SESSION] VARIABLES LIKE '%size%';
You can also filter the results of the SHOW query using a WHERE parameter as follows:
SHOW [GLOBAL | SESSION] VARIABLES WHERE VALUE > 0;