Call to undefined function mysql_connect() After upgrading PHP 7.1 to 7.3

I mean your php.ini is changed, check for the entry extension=php_mysqll.dll.

(If you has no knowledge where is your used php.ini – see https://stackoverflow.com/questions/8684609/dude-wheres-my-php-ini)

Is this entry active, has no ; before the string. After change of the php.ini you need a restart of mysql, like sudo /etc/init.d/mysql restart.

Also a note, check that mysql is installed for the newer php version, maybe via console or the phpinfo().

sudo apt-get install mysql-server mysql-common php7.3 php7.3-mysql