After pulling the latest xdebug source from GitHub and compiling, the generated xdebug.so has so many undefined symbols in it (complete output is attached separately):

nm -u xdebug.so | grep U
U dom_node_node_type_read
U dom_node_node_value_read
U dom_node_node_value_write

If we add -Wl,--no-undefined to CFLAGS, xdebug.so is not generated and several errors are issued.

These undefined symbols are defined in PHP source code (various modules), but are undefined in xdebug.so
When PHP-fpm is going to load xdebug.so as a zend_extension, it issues error and fails to load:

Failed loading /usr/local/lib/php/extensions/no-debug-non-zts-20180731/xdebug.so: /usr/local/lib/php/extensions/no-debug-non-zts-20180731/xdebug.so: undefined symbol: OnUpdateBool

Trying to add different .lo files from PHP directory to shared_objects_xdebug of xdebug's Makefile, does not solve the problem, as there are new undefined symbols or it leads to multiple definition error.

The issue does not seem to be related to PHP version, etc.