This is a very simple in-memory database, which has a very limited command set.

• SET [name] [value]: Set a variable [name] to the value [value]. Neither variable names or values will ever contain spaces.
• GET [name]: Print out the value stored under the variable [name]. Print NULL if that variable name hasn't been set.
• UNSET [name]: Unset the variable [name]
• NUMEQUALTO [value]: Return the number of variables equal to [value]. If no values are equal, this should output 0.
• END: Exit the program
• BEGIN: Open a transactional block
• ROLLBACK: Rollback all of the commands from the most recent transaction block. If no transactional block is open, print out INVALID ROLLBACK
• COMMIT: Permanently store all of the operations from any presently open transactional blocks

$ php cli.php 
SET a 10
GET a
UNSET a
GET a
END
-- OUTPUT --
10
NULL

$ php cli.php 
SET a 10
SET b 10
NUMEQUALTO 10
NUMEQUALTO 20
UNSET a
NUMEQUALTO 10
SET b 30
NUMEQUALTO 10
END
-- OUTPUT --
2
0
1
0

$ php cli.php 
BEGIN
SET a 10
GET a
BEGIN
SET a 20
GET a
ROLLBACK
GET a
ROLLBACK
GET a
END
-- OUTPUT --
10
20
10
NULL

$ php cli.php 
BEGIN
SET a 30
BEGIN
SET a 40
COMMIT
GET a
ROLLBACK
END
-- OUTPUT --
40
INVALID ROLLBACK

$ php cli.php 
SET a 50
BEGIN
GET a
SET a 60
BEGIN
UNSET a
GET a
ROLLBACK
GET a
COMMIT
GET a
END
-- OUTPUT --
50
NULL
60
60

$ php cli.php 
SET a 10
BEGIN
NUMEQUALTO 10
BEGIN
UNSET a
NUMEQUALTO 10
ROLLBACK
NUMEQUALTO 10
END
-- OUTPUT --
1
0
1