About the problem of sprintf intercepting decimals

Question: Keep 1 decimal place, do not round, fill in 0 when it is an integer

Many people say to use sprintf('%.1f',$str);, but there are still problems with the results.
Finally, this method was used to solve the problem: echo sprintf('%.1f', floor( $str));

Didn’t you say you don’t want to round? Why is it that when rounding, 5 is not included but 6 is included?
But other functions number_format($str) are all included at 5. Why is this?