It is a fairly common situation. Time ago, someone developed for you or your company a web application with PHP, which has barely needed maintenance and has been running for years, becoming a fundamental piece for your activity.
I'm talking about applications that work mainly under PHP 5.6, but it can also be with PHP 5.4, or even earlier. And including more recent applications, made for PHP 7, that, with the appearance of PHP 8, have also become obsolete, especially because hosting imposes using PHP 8.
The problem is delicate, because, especially in the case of versions prior to PHP 7, multiple critical errors will surely arise when trying to run the application under PHP 8. So, the scare is great, because redoing that same application now is going to require too much time and money.
However, you should know that it's possible to review that application to make it compatible with PHP 8, without needing to completely remake it.
Currently, PHP 8.2 is usually set as the target version. It's a matter of tweaking the application so that it does not generate errors running under this target version. However, retouching often includes improvements, such as encapsulating the interaction with the database if it is not already encapsulated (many PHP 5.6 applications use MySql functions directly instead of a class), and change the use of said MySql functions (obsolete) to MySqli or even PDO. Also, sometimes the configuration data is repeated in different files, so it's very convenient to centralize it in a single location.
I have a lot of experience in this type of tasks, and I'm used to applying reverse engineering on web applications made for PHP versions prior to branch 8.
If this is your situation, and you need my help, don't hesitate to write me telling me about the situation. Normally, after taking a look at the files to analyze the application, I will be able to offer you an approximate budget, which will undoubtedly be much cheaper than having to do the same application again.
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