How does shell_exec() pick which shell to use?

Question

The question came up in another thread. Figured I would ask.

Shell Script Loop from PHP not working

Answer 1

If it uses popen(), that uses /bin/sh on Linux. The man page for popen() says:

The command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to /bin/sh using the -c flag; interpretation, if any, is performed by the shell.

/bin/sh is the only shell that POSIX guarantees will be available.