Get WP Install Directory

The WordPress directory path is available in the constant ABSPATH. Be aware this is not related to the wp-content directory. The constant WP_CONTENT_DIR could be set to exactly the same value. Or another directory on the same level where WP_CONTENT_URL is another domain name.

There doesn’t even have to be a wp-content equivalent: themes, plugins, languages and so on might be spread over different directories.

Do not rely on default paths.

define('symbiostock_STOCKDIR', $_SERVER['DOCUMENT_ROOT'] . '/symbiostock_rf/' );

This will break in many setups. Use get_template_directory_uri() instead, and let WordPress figure out where that is.

Another word on constants: avoid them. There might be collisions with other people’s code. They are slow. Use a helper function to load classes instead, and solve all path problem in that function’s body, not in the global namespace.