General

Many thanks to the authors of the articles listed below, whose work has been invaluable when researching this subject.

Microsoft

• Parsing C++ Command-Line Arguments
• CommandLineToArgvW function
• What’s up with the strange treatment of quotation marks and backslashes by CommandLineToArgvW
• Everyone quotes command line arguments the wrong way
• Runtime argv parsing parse_cmdline source code.

Others

• Everything You Need to Know About Command Lines for Windows Programs
• A Better Way To Understand Quoting and Escaping of Windows Command Line Arguments
• How a Windows Program Splits Its Command Line Into Individual Arguments

The above three articles are part of an excellent series and include diagrams and sample programs.

• How Command Line Parameters Are Parsed
• How does the Windows Command Interpreter (CMD.EXE) parse scripts?
• How can I escape an exclamation mark ! in cmd scripts?

PHP

Documentation

• Program Execution
• Execution Operators

Source Code

• ext/standard/exec.c exec, system, passthru, shell_exec
• ext/standard/file.c popen
• TSRM/tsrm_win32.c popen_ex (called by the above)
• ext/standard/proc_open.c proc_open