wordpress、I couldn't log in after updating PHP!

I am also using WordPress.、The other day, I saw a message on my dashboard asking me to update PHP, so I decided to update it.。

- Requires changes on the server side
I'm a complete amateur。The moment I saw this、I was wondering if it would be changed somewhere else.、It was different.。I found out that I needed to make changes to the rental server I was contracted with.。

・Log in to Onamae.com for the first time in a while
The server I have a contract with is Onamae.com.。It seems that many people at Onamae.com have acquired domain names.、I am using a rental server.。
When I logged in for the first time in a while, I found the PHP update immediately.。When I looked at it, it was 5.6.40, so I changed it to 7.4.2.。

・Unable to log in、To restore it
I made a quick change and tried logging in to confirm.、What、

"A serious error has occurred"

The characters...what's this? ?
It's fixed。I wasn't sure, so I reset PHP to 5.6.40 and was able to log in successfully.。good…

no、It's not good! Could not update。I did some research on my own。Are the plugins not being updated?、Delete themes you don't use。

But I couldn't do it。It's in a state of disrepair。complete lack of knowledge。I'll do some research and try to update somehow.。I would like to report here again when I am able to do it! !