How to completely remove PHP?

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

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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

I do not recommend running sudo apt-get purge php*.

That was scary! Fortunately, I didn't type the -y option, because it chose about hundred of packages without php in their name.

sudo apt-get purge `dpkg -l | grep php| awk '{print $2}' |tr "\n" " "`

How it works:

First, a list of packages is generated using this series of commands: dpkg -l | grep php| awk '{print $2}' |tr "\n" " ".

Hint: You can run this part of the command in your terminal to see what packages would get removed. You should get something like: libapache2-mod-php5 php5 php5-cli php5-common php5-json

Finally, when you run the full command, this list of packages gets passed to sudo apt-get purge, removing all of the packages.

Hint: If it feels safer to you, you could just as easily run them separately, and copy+paste the list of packages to remove like so: sudo apt-get purge libapache2-mod-php5 php5 php5-cli php5-common php5-json

You'll probably want to purge all the php* packages from your system. Something with a wild-card should work

sudo apt-get purge php.*

You may be interested in How to rollback to PHP 5.2 for where to go next.