I have both PHP 5.6 and 7.4 installed on my server, because I still have several sites that use MySQL commands (which were removed in 7.x) instead of MySQLi; eg, mysql_query('whatever') instead of mysqli_query($dbh, 'whatever').

But... how about MySQL itself? I'm running v. 5.5, which went EOL in December 2018. Should I expect any problems (especially with the sites that use MySQL commands in PHP) if I update it to v. 5.7?

Note that I'm running tight on storage (using about 95%), and red lining the server's 4G of RAM from 6am to 10pm pretty much every day, so I'm hoping that these updates will be faster and more efficient.