Why can't i come back to older working version of php after ugrade with error log on php 7.2 and 7.3

Question

I'm getting locked out of */wp-admin/ there is an error log. Its after changing php 7.2 to php 7.3. The error doesn't disappear even if i go back to php 5.6 where MySQL_connect function exists and is used.

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in /home/punkmmur/public_html/wp-includes/wp-db.php:1564 Stack trace: #0 /home/punkmmur/public_html/wp-includes/wp-db.php(592): wpdb->db_connect() #1 /home/punkmmur/public_html/wp-includes/load.php(409): wpdb->__construct('punkmmur_calyps...', '7S4T]52!pg', 'punkmmur_calyps...', 'localhost') #2 /home/punkmmur/public_html/wp-settings.php(106): require_wp_db() #3 /home/punkmmur/public_html/wp-config.php(93): require_once('/home/punkmmur/...') #4 /home/punkmmur/public_html/wp-load.php(37): require_once('/home/punkmmur/...') #5 /home/punkmmur/public_html/wp-admin/admin.php(31): require_once('/home/punkmmur/...') #6 /home/punkmmur/public_html/wp-admin/index.php(10): require_once('/home/punkmmur/...') #7 {main} thrown in /home/punkmmur/public_html/wp-includes/wp-db.php on line 1564

Restoring website does not work. I disabled all plugins via phpmyadmin and by renaming folders in ftp. I changed theme to default too. Now even if i come back to my theme, some pages stay stuck on default theme even if i clear browser cache. Website is accessible: Tatouage Calypso, Tattoo Québec.I can't access wp-admin Maria DB is up to date. I tried multiple restores. Wp super Cache is deactivated i even trued to define to false the wp-cache entry in wp-config.php, i even added site url and home url in wp-config.php. I tried renaming .htaccess, i renamed theme directory to force fallback to default theme.

Answer 1

The problem was nd_mysqli php modules not activated for 7.2-7.3 but that still doesn’t explain why restore didn’t work once activated it worked but had some webmaster fix it for me. Maybe caching but i kept flushing caches.