Bug: getS7_Real() returns incorrect values for negative REALs (due to missing endian handling)

[flanguy]

Hello,

I've found a critical issue in the getS7_Real() function provided in s7_phphelper. The current implementation:

public static function getS7_Real(string $data, int $start)
{
    return self::hexToFloat32(bin2hex(substr($data, $start, 4)));
}

and

public static function hexToFloat32(string $strHex): float
{
    $v = hexdec($strHex);
    $x = ($v & ((1 << 23) - 1)) + (1 << 23) * ($v >> 31 | 1);
    $exp = ($v >> 23 & 0xFF) - 127;
    return $x * pow(2, $exp - 23);
}

produces incorrect values when decoding REAL values that are negative. This is because:

• It assumes the data is little-endian but Siemens PLCs (S7) store REAL values in big-endian format.
• It converts the bytes to hex and then to an integer with hexdec, which does not preserve the correct IEEE-754 format.
• This results in all negative REAL values being interpreted as large positive values, or garbage.

Solution

public static function getS7_Real(string $data, int $start): float
	{
		$bytes = substr($data, $start, 4);

		if (strlen($bytes) < 4) {
			return NAN;
		}

		// Siemens = big endian → use  "G" for float big endian
		$value = unpack("G", $bytes);
		return $value[1];
	}