reading and writing binary files

How can I create a binary file using PHP

ie I want to store say these information

1 bytes (record number)
10 bytes (name)


so say i have a record number that is represented by a byte followed by a name that is made up of 10bytes.

How would I create a binary file with this information? Seem like even though I use the"b" mode when openning files the record number gets stored as a string instead of a byte.

I can't use a text file for what I am trying to do.