Thanks everyone for your responses and suggestions. The problem has now been resolved.
Those quotes and italics were added by me after I copied the line.
Tim, you are perfectly right.
We tried to fix this issue "It looks like you are using a version of PHP that is so old that it does not recognise the array literal syntax ( [1, 2, 3] ) that was added in PHP 5.4."
In the process, the quote below was apparent.
"...So, probably a change was made to your side: someone changed the version of PHP being used to a very old one. That seems very strange."
That was exactly what happened. The hosting company did something wrong and unprofessional without informing us about it. Our sites were strangely running PHP 5.3. It took us a lot of effort and previous tickets to prove it to them that our server had been running on PHP 5.4 and that was why we were able to run moodle 3.0 before everything went wrong with all our sites running it.
Now all our moodle sites are running fine again as they have changed it to PHP 5.4.26.
Once again, thanks to everyone.