Ok. That's the story.
I don't think you should loose your time reading about my tries to use a different PHP Version. I really made a mess and now I switched back to the start point.
Includes all necessary compilation tools
sudo apt install build-essential
Stop your Apache server
sudo /opt/lampp/lampp stop
Back up your current XAMPP PHP files
mv /opt/lampp/php /opt/lampp/php.old
mv /opt/lampp/etc/php.ini /opt/lampp/etc/php.ini.old
mv /opt/lampp/modules/libphp7.so /opt/lampp/modules/libphp7.so.old
Create a tmp directory
mkdir tmp
Download a PHP version from the PHP website
Extract the contents of the compressed archive file and move all to the /tmp directory.
I am in /opt/lampp
mv /home/marco/Scaricati/php-7.3.0/* tmp/
Configure and build PHP in the usual manner.
I think it should take 3 steps: ./configure, make, make install
I made my tries, but I can't complete the configuration.
First I add --with-readline to the Configure Command and I got an error because of perl and, as I use XAMPP, I added a link
ln -s /opt/lampp/bin/perl /bin/perl
I got another error. After some reaserches
apt install libc-client-dev
Then the terminal asked me to add --with-kerberos and the Configure Command became this
./configure --prefix=/opt/lampp --with-apxs2=/opt/lampp/bin/apxs --with-config-file-path=/opt/lampp/etc --with-mysql=mysqlnd --enable-inline-optimization --disable-debug --enable-bcmath --enable-calendar --enable-ctype --enable-ftp --enable-gd-native-ttf --enable-magic-quotes --enable-shmop --disable-sigchild --enable-sysvsem --enable-sysvshm --enable-wddx --with-gdbm=/opt/lampp --with-jpeg-dir=/opt/lampp --with-png-dir=/opt/lampp --with-freetype-dir=/opt/lampp --with-zlib=yes --with-zlib-dir=/opt/lampp --with-openssl=/opt/lampp --with-xsl=/opt/lampp --with-ldap=/opt/lampp --with-gd --with-imap=/bitnami/xamppunixinstaller72stack-linux-x64/src/imap-2007e --with-imap-ssl --with-gettext=/opt/lampp --with-mssql=shared,/opt/lampp --with-pdo-dblib=shared,/opt/lampp --with-sybase-ct=/opt/lampp --with-mysql-sock=/opt/lampp/var/mysql/mysql.sock --with-mcrypt=/opt/lampp --with-mhash=/opt/lampp --enable-sockets --enable-mbstring=all --with-curl=/opt/lampp --enable-mbregex --enable-zend-multibyte --enable-exif --with-bz2=/opt/lampp --with-sqlite=shared,/opt/lampp --with-sqlite3=/opt/lampp --with-libxml-dir=/opt/lampp --enable-soap --with-xmlrpc --enable-pcntl --with-mysqli=mysqlnd --with-pgsql=shared,/opt/lampp/ --with-iconv=/opt/lampp --with-pdo-mysql=mysqlnd --with-pdo-pgsql=/opt/lampp/postgresql --with-pdo_sqlite=/opt/lampp --with-icu-dir=/opt/lampp --enable-fileinfo --enable-phar --enable-zip --enable-intl --disable-huge-code-pages --with-readline --with-kerberos
Then
apt install libkrb5-dev
And then it asked me where is postgresql, so I removed --with-pdo-pgsql=/opt/lampp/postgresql and --with-pgsql=shared,/opt/lampp/
Now it asks me to reinstall readline.
That's enough!
I summarized the story.
I'm going to keep tinker as it is.