fatal error FATAL ERROR - What is meaning of this message?

[katicziv]

Fatal error: Call to undefined function mysql_connect() in /public_html/......./classes/db/MySQL.php on line 51

Is this mean no connection to database and where to point attention to solving this problem?

BO is working perfect, FO is not work

DATA BASE IS CHECKED, NO ERROR 
https://www.screencast.com/t/VTq0JbAf4j

MySQL.php on line 51
https://www.screencast.com/t/OhN8FV4g

settings.inc.php 
server / user / password, have been entered correctly.

Configuration Information
https://www.screencast.com/t/C7J3VFTDK5w

where to point attention to solving this problem?
I didn't find anything on forum / net

When that happened, nobody did anything in BO. 
but it happened on two different domains, at the same day (second domain is 4 weeks old copy of first domain)
Have more prestashops on same server and only these two domains have same error.

Edited April 21, 2018 by katicziv (see edit history)

[selectshop.at]

Which Prestashop version ? which php-version ? which mySQL version, or are you using mariaDB ?

Check also the same problem on this thread: https://www.prestashop.com/forums/topic/544740-solvedprestashop-1616-not-work-with-php7/

[razaro]

Check configuration screenshot, PHP is 5.4 bit old but if that worked before it should not be an issue.

 

Though @katicziv you can try to change to PHP 5.6. You have debug mode enabled I guess. But can you check server logs or

log folder in root maybe for more detailed stack trace of error.

Have you updated any modules recently ?

[selectshop.at]

@razaro - Oh sorry, yes I missed second picture. This is very old php-version.

 

[katicziv]

I change PHP to
5.6
5.5
and when I back to 5.3 everything back to normal

SOLVED