I have the same problem when I want to extract the value of a LONGBINARY field from an MS-Access or MSSQL Database (OLE Field exactly), and the OLE embeeded header contains some zero bytes. I can obtain only 3 bytes when it converts to a string for save to a file. And when I loop with "foreach" the VARIANT ARRAY of UNSIGNED BYTES it loops correctly (except for the byte '\0' after every valid byte) and if I count the array with "count($field->Value)" for example, it results the double lenght that expects.
Why don't exists in PHP a way to obtain a simple array of bytes for a variable, or access the data like a "string" of bytes?

The problem is that php_com_olestring_to_string()[1] is not binary
safe, because it passes -1 to the cchWideChar parameter of
WideCharToMultiByte()[2]. That causes WideCharToMultiByte() to
stop at the first null character (actually, two null bytes), and
so the rest of the string is ignored.

As a BSTR[3] has a prefix containing its length in bytes it
appears we could use this to calculate the proper cchWideChar
argument, see the attached patch.

[1] <https://github.com/php/php-src/blob/php-7.0.0beta3/ext/com_dotnet/com_olechar.c#L74>
[2] <https://msdn.microsoft.com/en-us/library/windows/desktop/dd374130(v=vs.85).aspx>
[3] <https://msdn.microsoft.com/en-us/library/windows/desktop/ms221069(v=vs.85).aspx>