How to write binary data to a file?

Hi all, All I can find in the PHP Manual for writing raw bytes (unsigned integers) to a file is fwrite. But when I use it, it seems to write characters, not raw bytes. For example, the value 1 (0b1) is actually written as 0x31, not 0x01 as desired. Yes, the fopen was done with the "b" mode specified. It doesn't work this way in C or C++, and the PHP Manual says nothing about this. Web searches turn up nothing useful on this subject. I have posted this question on three PHP fora, and have received only unhelpful or irrelevant answers so far. System: Windows 8.1, PHP 5.5.10