This method is not advised to be used without careful review. Read below comments.

This should completely remove any package with a name that starts with php and anything related to it.

sudo apt-get purge 'php*'

DON'T PRESS y UNTIL YOU ENSURE that in the removing packages list there are no other packages (besides related to php packages), like:

php-common* python-openssl* php-curl* ... and tons of packages

If so, type n, copy the list (& tidy up from the unrelated packages), and manually remove them:

sudo apt-get purge      php-common* php-curl*    ... and tons of packages