A lot of the people I host are not coders. They don't understand things like php, globally scoped variables, deprecation warnings, database authentication plugins, insecure hash types, or the like.
They know only that they have code that worked fine for a decade, and then Some Jerk Ferrit did something that made their site not work.
Most of this is because PHP, as a language, is a Shit Show. The only reason PHP scripts are not still majorly responsible for most of the botnet activity on the internet is because someone decided to make smart light bulbs with globally routable ipv6 addresses.
Coding in php is like trying to sculpt something in clay, except that people keep dumping ingredients in the clay that change its consistency: sand, water, cement, cheerios.
For an admin, php is a security nightmare: you have 300 users, whose code can all alter each others' files. Oh, and on most webservers? Users can't alter the files PHP created. They're owned by the "www" user.
So, because vague reasons, the people who make the PHP language decide that a particular function is not workable in the particular coding style that they feel people should be using at that time. So, somewhere in a README file that nobody actually reads, they say "hey, you should stop using this function, it may go away in the next version".
I hosted several hundred websites at one time -- nobody knew about that README file, which, as far as they knew, were on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying 'Beware of the Leopard.”
So, a long long time ago, I solved two birds with one stone. I installed a program called "suPHP". What suphp brilliantly does, is sacrifice some of the speed normally present in PHP, by running everyone's PHP scripts as them. It does this by decoupling PHP from the webserver, and winding up a tiny little PHP process to spawn your files.
The unexpected side effect here, is that it can run different versions of PHP for different users.
Now, as far as the operating system is concerned, you can only install packages for one version of PHP at a time, and right now, at the time of this writing, that's PHP56, with a bunch of removed functions and deprecation warnings.
I've been building PHP from scratch for years, tho, and I know how to install a tiny little shadow copy of an older version of PHP where the webserver can get at it.
In fact, I even have a separate copy of apache running with the mod_php going on, for my webmail, where I can use the speed.
Best part? You can control it.
If you were to look at this htaccess file, you can see how easy it is to signal to the interperter that you want 5.5. (Normally, apache won't serve .htaccess files out to the world, this one is special). Basically three lines of code:
<FilesMatch "\.php$"> SetHandler application/x-httpd-php55 </FilesMatch>
In a former life, I let people use this to switch between php4 and php5. RIght now the only handlers are php5 and php55. I could maybe add php54 as well.
That said -- if you possibly can, I advise using upgraded code that supports the latest thing. So if you're running something like Wordpress, please do update. If on the other hand, you have an old copy of Gallery, and it's not being hacked or hammered, and it suddenly broke, the above will fix it.