Using sprintf with a floating point value for a negative number leaves a null character in the string, such as:
$num = sprintf("%0.1f", -12.5);

$num will contain the following characters within the actual string when printing out:
0x45 0x31 0x32 0x2E 0x35 0x00

This does not occur with non-negative numbers.

Try the following for realtime example:
<?php
$num = sprintf("%0.1f", 12.5);
echo "Positive Num: $num<br>\n";
$num = sprintf("%0.1f", -12.5);
echo "Negative Num: $num<br>\n";
?>

This is apparently an old bug that's been reintroduced in version 4.3.2.