Using random in php mysql can't random id?

You should use ORDER BY RAND() and not ORDER BY RAND(prod.product_id)

From RAND manual:

RAND(), RAND(N)

Returns a random floating-point value v in the range 0 <= v < 1.0.
If a constant integer argument N is specified, it is used as the seed value
which produces a repeatable sequence of column values.

The argument inside the RAND(x) function is the seed, so you're seeding it with the same value every time. Instead, leave it as RAND() and it will be different.

SELECT id, name FROM products AS prod ORDER BY RAND() LIMIT 5

You probably want just ORDER BY RAND() rather than ORDER BY RAND(prod.id) -- if you give RAND() an integer argument, it's used as the seed value, so you will always get the same "random" value back.

See http://dev.mysql.com/doc/refman/5.0/en/mathematical-functions.html#function_rand for the details.

Use this way:

SELECT id, name FROM `products` AS prod 
ORDER BY RAND() LIMIT 5;

See manual here