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));

<code><?php
    $str1 = 12.34;
    $str2 = 12.35;
    $str3 = 12.36;
    echo sprintf('%.1f',$str1);//12.3
    echo sprintf('%.1f',$str2);//12.3
    echo sprintf('%.1f',$str3);//12.4
    
    echo sprintf('%.1f', floor($str3));//12.3
?>
</code>

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?