in php.ini you can reference to an existing directive or an environment variable using the syntax ${varname}.
Here are some examples.
sys_temp_dir = "${WINDIR}"
--- ${WINDIR} will be replaced by $_ENV['WINDIR'] at runtime
--- you can set environment variables by Apache and use them in php.ini
--- FcgidInitialEnv AUTHOR "WUXIANCHENG"
--- error_log = "${AUTHOR}.log"
error_log = "${sys_temp_dir}"
--- ${sys_temp_dir} will be replace by the value of sys_temp_dir
Also you can use PHP constants in php.ini, but DONT'T wrap them in ${} or "".
error_log = "/data/"PHP_VERSION"/"
--- it works like this php code:
$error_log = "/data/" . PHP_VERSION . "/";