How to round a number to significant digits?

Question

PHP

How to round a number to significant digits?
I don’t quite understand why, but numbers are stored in the database (type double)
And PHP (MySQL?) Extracts some (not all) incorrectly
For example, there is a number in the database - 69.6
It is extracted as 69.59999999999999
Or the number 8.3
is Extracted as 8.30000000001
Where do these insignificant numbers?
Is it some kind of setting in PHP or MySQL?
How can you normalize numbers?

Answer 1

SQL floating point comparison issues

SELECT ROUND(8.30000000001,2);
-- 8.30

$a = round(8.30000000001,2);
// 8.30

Answer 2

In general, change the data type to DECIMAL .
In private, for example, with currencies, switch to integers, that is, instead of rubles, count in kopecks or fractions of kopecks.