HTTP Error 500

Hello there, I have problem accessing one of my web sites. I think something with the mysql.php version, not sure acctualy. But I have another web page build with other e-commerce platforms which gets the nect error: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in /home2/itscomp1/public_html/bebecomfortco/system/database/mysql.php on line 6

I have try fixing but the the end of error is never end. Any advice what I can do?
Thank you in advance.

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